Building a Spectrophotometer

In the autumn of 2025 I attempted to build a spectrophotometer by myself.

A spectrophotometer is a scientific instrument that measures the amount of electromagnetic radiation, or light as it is commonly known, that is absorbed by a sample. As different molecular bonds absorb light at different wavelengths, the absorption of light says something about your molecular composition. The most practical use for this is the determination of the quantity of a known substance in a sample.

In order to measure the absorption accurately, the light that passes through a sample is ideally only comprised of a single wavelength. This is a major difficulty in the design of the instrument, which can be overcome by something called a monochromator, of which the Czerny-Turner monochromator is the most common design.
In a Czerny-Turner monochromator, light from a white light source is aimed a concave mirror, which sends the light towards a movable grating that diffracts the light and breaks it up into individual wavelengths. These are then focussed by another concave mirror and aimed at a narrow slit, which in theory only lets one wavelength of light through. This light then passes through the sample and the reduction in intensity of the light is measured:

So while the principle of a spectrophotometer is simple, manufacturing its parts to analytical standards requires high precision, and therefore these machines are expensive. Brand new instruments are several thousand euro's and even decades-old equipment can still fetch prices of several hundred euro's. For example, this beauty from the 1970's is listed for 500 euro's today:

Because its principles are relatively straightforward and can be observed with the naked eye, the spectrophotometer is often an early introduction to scientific equipment within an educational context. Indeed, many teaching kits are commercially available, to make it possible to see the inner workings of the instrument and freely manipulate the individual components around. However, such teaching kits still aspire to the same level of quality as the commercial equipment and so the prices are still high, often exceeding a thousand euros for a basic model.

Due to the accessible nature of the machine's workings, there have also been many published instances of spectrophotometers built out of simple(r) materials and on a small(er) budget. Examples include Peiera, et al. (2019), Kovarik, et al. (2020), Shin, et al. (2022), Osterheider, et al., (2022), and Poh, et al. (2021).
However I noticed a pattern in these suggestions. They tend to be either limited in functionality, restricted to light of a single wavelength, or are only applicable to a small number of known analytes.
The more general purpose designs I've encountered on the other hand tend to incorporate at least one 'cheap' component that is nevertheless a considerable expense, such as a professional grade grating mirror, access to a 3d-printer, or a smartphone equipped with a camera. While such items are somewhat commonplace, if one has to purchase one specifically for the project, it quickly drives up the cost to 100+ euro's. 

With this in mind I went to make a spectrophotometer of my own design, based on the Czerny-Turner monochromator. My first goal was to make a functional general purpose spectrophotometer and the second goal was simply to spend as little money as possible.

In order to achieve this I aimed my attention at the cheapest materials I could think of that could perform the required function in my design.
For the monochromator I therefore used a rechargeable LED-flashlight as the light source. The price of the flashlight was seven euro's. Its light was reflected by two plastic make-up mirrors and broken up with a cd, for a total cost of another seven euro's. The whole thing was made of recycled wood, with the slit being a cut in a thin piece of veneer, attached by some tape. The wood consisted of scrap material from other projects, but let's value it at a generous five euro's.
The detector consists of an Arduino board with a € 0,40 phototransistor and a two-line lcd-screen. Together with a breadboard and some other bits and pieces this came in at a total of € 15,05.

The total cost of my spectrophotometer, if one had to build it from scratch, thus came in at 34 euro's and five cents. For this money you get a design that is compatible with standard (disposable) cuvettes that are used throughout the industry:

And an overhead view of the instrument in operation:

Of course the instrument I built is not plug and play and there are a few things I learned about its limitations.
The light yield is low due to the low quality of the mirrors that lack a uniform focal point. Therefore the amount of diffusion is high towards both ends of the visible spectrum, making the instrument the most effective in the green to orange colour range.
I also found out that a phototransistor was much more sensitive in this case than a photoresistor, and the transistor also had a more consistent output throughout the whole spectrum, while the sensitivity of the photoresistor I tried was greatly reduced above ~600nm.
Also the slit in the veneer is still somewhat broad, even if it was cut with a sharp scalpel. Therefore one can only measure the absorption in a broad-ish range of about 50 nm instead instead of a single wavelength.

In terms of its practical use, the calibration calculations have to be performed by hand. First the maximum absorption of the sample is determined, before a blank and a series of standards are measured against that point of maximum absorption.
As there is no (reliable) way to record this specific maximum, repeatability of experiments is a possible issue, as the measurement cannot be repeated exactly with known wavelengths.

Nevertheless, I found the instrument to be accurate and reliable with solute concentrations as low as 1 mg/mL. This is much less sensitive than commercial models, which can often reliably detect concentrations of 1 mg/L or even lower, but it's perfectly useable for my personal applications.

By not attempting to adhere to modern analytical standards, I have thus been able to build a functional general purpose spectrophotometer compatible with standard single-use cuvettes for about the same price as a single package of these cuvettes.

Middlemen

In both art and science, the products of (small groups of) individuals are disseminated to the world by other companies. In the world of art, these companies are the galleries representing artists. In the world of science, they are the publishers and their journals.
In both these situations there is a clear distinction between those who produce the goods and those who distribute them to a wider audience. The presence of such middlemen is common in many industries, but an uncommon aspect found in both art and science is that the financial benefits to the intermediary are far greater than those of the producer. Scientific publishing is now a multi-billion euro industry and the largest of the art galleries have turnovers in the range of tens of millions of euros.
The curious similarities between the two fields are the result of imperfect information on the consumer side, combined with some leftovers from an older world where the financial risks were differently distributed and legally arranged. 

For both scientific publishing and art galleries the most valuable asset is the firm's reputation.
For example, the price of an artwork is linked quite directly to the standing of the gallery it is shown in. Similarly, a scientific discovery is generally considered more impactful if it's published in a journal of significance. It's therefore imperative for both galleries and scientific journals to become, and remain, reputable. It's also easy to see that for both fields there is simultaneously no inherent and necessary connection between the quality of the work and the social standing of the middleman. The intermediary does not change any intrinsic property of the final product. That the perceived quality of the intermediary is nevertheless seen as a useful indication of the quality of the good is due to a characteristic that economists call imperfect information.
In both art and science, there is no information about the quality of a good that is both reliable and readily available. The causes of this imperfect information are different in each field, but over the course of the last century they have led to a similar outcome where the intermediaries have a disproportionate influence on both the kind of goods that get produced as well as which consumers have access to it.

Any consumer needs information about a good in order to make a decision about what is worth spending their money on. They can either have full access to all necessary information, which is called perfect information, or limited access to one or more characteristics of the good, which is called imperfect information.
In both art and science, information about the quality of a good is difficult to ascertain for a large number of interested buyers. Quality in the arts is next to impossible to quantify and subject to changing cultural perceptions. And while scientific merit can be checked in principle, this requires an impossibly large amount of time, money and other resources, so in reality it is unfeasible for any one party to make an objective judgement based on their own experimental knowledge about the quality of all articles published in all journals.
Hence, for both art and science, there is a lot of effort that goes into convincing a potential customer of the value of the good that is being sold. As the goods themselves don't provide accurate clues to their genuine value, this is done through less direct means that convey a perception of longevity. Such (signs of) longevity can ostensibly only be reached by consistently providing quality goods.

Galleries and artists today try to provide credible signals of value by demonstrating a long-term commitment to each other. Until the early 20th century, this meant that dealers were directly buying (nearly) all of an artists output, thereby putting their money where their mouth is. If nothing else, this would at least demonstrate to a potential client that the dealer has a strong belief in the artist. And the dealer is only able to aquire those works today if they have made sound financial decisions in the past. These days the commitment is less strong, expressed by 'representation' of the artist by the gallery. The financial capabilities of the gallery are generally demonstrated through large, and mostly empty, spaces in expensive buildings in desirable locations, as well as participation in ludicrously priced art fairs.
I've written elsewhere on this blog how this shift is likely partially caused by changes in anti-trust legislation in the Western world in the first half of the 20^th century, so I won't delve any further into this subject here.

The publishing of scientific works likewise underwent significant changes in the last two centuries.
A scientist is usually employed by a university or some other institution, and when they've made a discovery, they write up what they've done to try and make it known to others what they have discovered. It is of course difficult to reach a broad audience, even in the age of the internet, so this is one thing a publisher can help you with. A publisher possesses ways to reach an audience that any single scientist doesn't have. A publisher also has access to infrastructure. Although these days more and more of scientific publishing is done digitally, physical printing and distribution of materials has historically been a venture with large upfront costs, combined with specialised knowledge and equipment. These upfront costs carry significant financial risks, which can only be borne by a large company that is able to spread such risk over multiple ventures. 

It's also a well known fact that most scientific literature has a very small readership. Current estimates on the audience size of the average journal article range from single to triple digit numbers.
Yet at the same time, there is a great number of scientific articles that are published every day. With such fragmented readership, there is little possibility for scientific texts to gain widespread attention in the same way a newspaper article or a viral video might. Therefore, to a broad audience it is virtually unknown what the value is of any given article relative to all the other articles that are available.
As already stated, some of this uncertainty is remedied through the reputation of the journal the article is published in. This reputation is mostly based on the reception of the works that were published by the journal in the past, as well as the academic standing of its current editor(s). There have been attempts at quantifying this reputation by metrics like a journal's impact factor, which essentially measures how often articles from a journal have been cited by other scientists. But as Goodheart's law states, any measure that becomes a target seizes to be a good measure, so such undertakings merely repackage the problem instead of solving it. 

Both industries thus have a small customer base and these customers ought to be sceptical of the goods they provide and the high prices they ask for them.
So how do these middlemen leverage their position to create profits for themselves?
In the arts it is a simple question of gallerists charging very high commissions for their work, so that a handful of sales can provide an adequate amount of turnover, especially when their risk is spread over a number of artists.
In scientific publishing, exorbitant profit margins only arrived around the turn of the millennium, and to see why this is the case requires a short history lesson on copyright law, and in particular how such laws were implemented in the United States of America.

The foundation of today's copyright legislation was laid at the Berne Convention in 1886. This type of copyright is based on an idea of author's rights, where the creator of intellectual property is also automatically the owner of intangible rights relating to that work. These reproduction rights could then be licensed to a third party, such as a publisher. This can happen in different ways, but it must be noted that a perpetual exclusive license to reproduce the work is an option, even when the author retains the copyright in such a case.

This is in contrast with the common law idea of copyright, which is much more focussed on the economic right to publish and distribute. The United States, which legal system is based on common law, was thus late in incorporating the principles of the Berne Convention. In the early 20th century, copyright for individuals did not exist, but a publisher could register the publication of a work at the copyright office to obtain its copyrights.
It's a bit of an oversimplification, but it wasn't until the Copyright Act of 1976 that the intellectual property laws of the United States became more closely aligned with those of most other countries.

This change has quite directly lead scientific publishers to mandate their authors to sign over their copyrights to the publisher, instead of licensing their papers. At best this can be seen as a good-natured attempt to retain the best publishing standards possible, but it's much more likely that this decision was aimed at retaining control over the substantial captial the publishers had ammassed up until that time.
For example, in the 1966 edition of the Handbook for Authors of Papers in the Journals of the American Chemical Society, the section on 'Liability and Copying Rights' is only half a page long and simply states 'The Society owns the copyright for any paper it publishes'. This was true under the federal copyright laws of the USA at the time, which required registration at the copyright office.
Interestingly, the section on 'Liability and Copying Rights' of the 1978 edition of the Handbook for Authors of Papers in the Journals of the American Chemical Society was nearly twice as long as the previous edition. It now contained the following phrase: 'Under the terms of the Federal copyright law, effective January 1, 1978, scientific publishers who wish to obtain copyright ownership of papers in their journals are required specifically to obtain such ownership from the author of each paper. Since it is necessary for the widest possible dissemination of scientific knowledge that the society own the copyright, authors are required to transfer copyright ownership before publication of their manuscript.'

This last sentence is simply not true. A perpetual, even exclusive and non-restricted, license to publish poses no practical objections. However, such a license would still leave the ultimate ownership in the hands of the author, so that the publisher could not license the work out to third parties. Transfer of copyright ownership thus is an issue of control of the work beyond the any direct publishing efforts in their own journals.

However, it might have been vital for publishing companies to protect those interests. In the 1970's, publishing was still a complex and costly business, with large upfront costs and little or no guarantee that anybody would be interested in the final product. The publishing industry had a high risk of failure and the small number of scientific publishers that have survived, only survived because they originally published books that turned out to be of particular significance and relevance to other scientists. Unlike most of their publications, these tomes had several reprints and made a healthy profit, which could offset the cost of the many failures.
It is, however, impossible to predict which publications become a hit, and if the publisher didn't own the copyright, such a reprint would have most likely have to be renegotiated with the author. This author by then has of course seen how well their book is selling and so they'll likely want a bigger cut for themselves, or could even let the reprint be done by a different publisher altogether. This is therefore debilitatingly risky to a publisher during that time, and so the transfer of copyright ownership may have been a reasonable request in the 1970's. 

This all changes towards the 2000's and the advent of widespread internet access. Through more than a century of publishing and consolidation in the industry, a handful of scientific publishers are in possession of enormous archives and because of their insistence on copyright ownernship, they have full control over them. Through digitisation these materials are now also easily searchable and through the internet they can be distributed at negligible cost to the publishers. In other words, the publishers' material capital is now more valuable than ever, while their operating costs have fallen dramatically.
As a result their profits have risen to extraordinary heights. To illustrate this fact, two of the top ten highest paid CEO's in the Netherlands are scientific publishers. 

In summary, the presence of middlemen is necessary in both art and science to create credible signals about the quality of goods. And in both these markets, the middlemen have understood the necessity of their presence and found ways to leverage that power into great profits by essentially exploiting the weak negotiating position of their suppliers and in some cases those of their clients.
Such predatory practices are much lamented in both industries, yet I'm unaware of any proposed solution that could remedy the problem. Many of such initiatives are focused on the (financial) inequality of the artist/scientist and gallery/publisher, but I believe a solution can only be found in making information about the quality of goods readily available to end users.

As a final remark, it must be noted that book publishing in the arts is a market that functions remarkably well, considering the difficulties that exist in scientific publishing and the sale of artworks. In art publishing, there is a healthy market of buyers and sellers, while risks and profits are usually shared in reasonable terms between the artists and the publishers.
The reason for this is simply that an art book can quite literally be judged by its cover. When selecting which art book to buy, an interested buyer usually can find the books that appeal to them by considering the design of their covers. Art books also retain their value rather well, so that even if a mistake is made, a buyer can still resell the book at only a minimal loss. Therefore information on the quality of a good is widely available, while the cost of misinformation is marginal.
This is the exact opposite of scientific publishing and the market for artworks, where credible information is hard-won and the costs of getting it wrong can be extremely high.

Contributing Factors in a Cheerios-based Adhesive

One day, a few years ago, I ate a bowl of cereal and I haphazardly forgot one piece of cereal in the bowl. I then also neglected to wash the bowl for a number of days, causing the milk to dry out. When I picked up the bowl again, I noticed that the piece of cereal, Cheerios, was stuck firmly to both the spoon and the bowl.

Being interested in the potential of such a Cheerios-based adhesive, I decided to 'glue' a spoon to a window by dipping the Cheerios in milk and clamping it between a spoon and a piece of glass:

 Seen from the side it would look like this:

I didn't really have any idea of how it worked at the time, but knowing that both metal and glass are two materials that are often difficult to stick together, it was striking to me that Cheerios, when combined with milk, would be able to act as an adhesive for these two objects. 

I never quite figured out how to clearly show that it was in fact the combination of milk and Cheerios that kept the spoon in its unusual place, so it never went very far as an artwork.
However, it still intrigued me from a chemical point of view, as it was an odd combination of materials to be stuck together so easily. Adhesion of such dissimilar materials is often very much influenced by mechanical adhesion. In mechanical adhesion, the (invisible) roughness of a surface is filled up with a material that is liquid at first but then hardens to a solid. These two materials are then not chemically bonded together, yet they can't move as there is no physical space to do so. 

It was however unlikely that this is the full story in this particular case, as both glass and metal have relatively smooth surfaces and nothing in milk actively polymerises as it dries. There are therefore very few cavities to fill and no obvious substance to fill them with.

Being interested in surface interactions for another project, it occurred to me that the electrostatic activity on the surface of the metal, combined with the free electron pairs in the silicon dioxide of the glass, could perhaps create non-trivial hydrogen bonds with the sugar molecules in the milk. The Cheerios are in turn largely comprised of long chains of sugars, so that the sugar from the milk can form hydrogen bonds with those and possibly have an intertwining crystallisation structure, providing rigidity. 
This combination is partly illustrated in the following diagram, where (1) denotes the crystal lattice of the metal and the free electron pairs on the surface, (2) are hydrogen bonds with the sugar molecules, (3) are the sugar molecules that are left over when the water has evaporated from the milk, (4) are the hydrogen bonds between the sugar from the milk and the polysaccharides from the cereal, and (5) are those polysaccharides.

 

To test the plausibility of this hypothesis, I devised several experiments where different combinations of materials were tried out iin order to isolate and test a number of variables.

For these experiments, single Cheerios were placed in liquid and left to soak for 30 minutes. The liquids used were semi-skimmed milk, water, or water with an amount of sugar dissolved in it.
The wet Cheerios were then placed on a glass or plastic surface, and a spoon was placed on top. The spoons were balanced so that their own weight pressed upon the Cheerios.
This was then left to dry for ~3 days.
The degree of adhesion was then determined by the experimenter through detaching the materials from each other. This could result in either low, or no, tack (denoted as --), some tack (denoted as +/-) or high tack (denoted as ++).

The results of the various experiments can be found in the following table:

Materials	Result, Expected	Result, Observed
Glass, Spoon (std), Cheerios, Milk	++	++
Glass, Spoon (std), Cheerios, Water	--	--
Glass, Spoon (std), Cheerios, Sugar Water	++	++
PolyPropylene, Spoon (std), Cheerios, Milk	--	--
PolyMethyl MethAcrylate, Spoon (std), Cheerios, Milk	++	+/-
Glass, Spoon (smth.), Cheerios, Milk	+/-	++
Glass, Spoon (std.), Milk	+/-	+/-
Glass, Spoon (std.), Sugar Water	+/-	--
Glass, Spoon (std.), Kitchen Paper, Milk	++	-- & ++

The expected result was the result based on the theory as outlined above and the observed result is what actually was the case. It's clear to see that the expected and observed results match each other closely.
There were a couple of instances where the observed result differed from the expectation, however, namely in the case of the PMMA substrate, a smooth metal spoon, sugar water in the absence of cereal and the substitution of Cheerios for kitchen paper.

The observation that there was high tack in the combination of Cheerios with both milk and sugar water, while there was no adhesion at all when the Cheerios was only soaked in water, shows that the presence of sugar is very important in the adhesive properties of this combination of materials.
That the Cheerios with milk showed high tack on glass, some tack on PMMA and no tack on polypropylene also indicates that hydrogen bonding is very important to the adhesion to the glass substrate, as was expected.

An experiment done with a spoon that had a very smooth surface also shows that the observed adhesion is chemical or electrostatic, rather than mechanical, in nature. It was expected that a smoother surface would give less adhesion to the spoon, yet no discernible difference was observed between a well-used spoon and a new, smooth, spoon.
Two experiments performed with only milk and sugar water in the absence of Cheerios showed that sugar alone can't act as an effective adhesive for these materials. While the sugar stuck firmly to the glass, likely through hydrogen bonding, it showed virtually no adhesion to the metal spoon. Nevertheless, a thin droplet of milk did have some tack to the metal, so that some other component of the milk must be the substance that binds to the metal. The most likely candidate is calcium, as calcium ions are very large and able to form complexes with a high coordination number, thereby binding various molecules together.

To examine the influence of the Cheerios, an experiment was performed where a wad of kitchen paper, made out similarly long polysaccharides, was soaked in milk.
This gave an interesting result, where this wad strongly adhered to the glass, but showed no tack on the metal surface. This is most likely caused by the greater absorbance of kitchen paper, so that the sugars or ions in the milk where in little contact with the metal as the water evaporated.

In conclusion, when using Cheerios and milk as an adhesive for metal and glass, all four components are important contributors to the overall effect. A major contributor to the adhesive strength is the large amount of sugar found in milk, which is aided by other components, where an abundancy of calcium likely aids in bonding to the metal of the spoon. The combination of milk and Cheerios binds to the glass through hydrogen bonding and to the metal by some other chemical or electrostatic force, where mechanical adhesion only has a limited contribution.

Party Pooper

The above photograph comes from the series 'The Action of Matchmaking Photons in Bars' by Voebe de Gruyter. In a conversation with Maria Barnas, titled 'On Art and Science', she says the following about it:

'The.photos I took in the café are real spots of light. I had stuck reflective tape on the people and the interior and took pictures using the flash. I see the spots of light as proof of light's return.'

The premise here is that light originates from the flash, hits an object and then returns to the camera lens and its sensor, rendering the image. But if we assume that this is the case, as the artist does, then the rest of the photograph, or even any photograph taken with a flash, surely is an equal 'proof of light's return'?

Methyl Mercaptan

Artists like to use molecular models for making sculptures. This has already been covered on this blog, but I'd like to expand on the subject a little further in this post.
Molecules have certain stable configurations, which are governed by the distribution of their electrons. This is described by something called valence shell electron pair repulsion theory. It's somewhat complicated, but just imagine that electrons are magnets on a sphere that want to be as close to the centre as possible, while being as far apart from each other as possible. So while atoms are always in motion, this means that on average they are found in only a small number of configurations in molecules:

 

 

This kind of spatial configuration is correctly rendered in the large sculpture 'Gas Molecule' commissioned from Marc Ruygrok by the NAM:

This sculpture is supposed to depict methane, or CH₄, with a central carbon atom connected to four hydrogen atoms. Ruygrok has largely copied the common 'ball-and-stick' molecular model, only taking some liberty with the colour scheme.
Although molecules don't have a 'real' colour, there is a convention, called Corey-Pauling-Koltun colouring, for using certain colours for certain atoms. The central atom in Ruygrok's model is carbon, which in this convention is always associated with black, while blue is always associated with nitrogen. If the shiny purple-ish hue of the central atom is considered significant, then this is traditionally linked to phosphorous, but is today more commonly associated with potassium.
These colours are nothing but conventions, so it's not that Ruygrok's choice is wrong per se, but it also isn't 'right' to use blue in this case. Without any other information, any chemist will think this model represents ammonium, not the intended methane.

As already stated, this example uses the so-called ball and stick model, but a more realistic space filling model exists where atoms are depicted as overlapping spheres representing their Van der Waals surface. Molecules in this model consist of interconnected spheres, so that a good separation through size and colour becomes even more important than it is in the ball and stick model. With this in mind, let me present to you 'Calcium 4-[4-(2-methylaninlino)-2,4-dioxobutyl]diazenyl-3-nitrobenzenesulfonate (C.I.13940)' by Jean-Luc Moulène:

This is supposedly a model of the molecular structure of a pigment, Yellow 62, which is then painted in the colour of this pigment. I already pointed out that without adequate differentiation through colour, such a model is hardly able to serve its clarifying function.
It is however clear that Moulène didn't correctly render the molecule he meant to render. When I looked up and drew a model of the pigment, I came up with the following structure:

Even without knowing anything about chemistry, it's obvious that these are are two different structures. In the correct model, there are 41 spheres present, while in Moulène's sculpture one only counts 29 spheres. I did notice that in Moulène's sculpture no hydrogen atoms were depicted, which is somewhat common practice. I therefore counted the amount of hydrogen atoms that should be present, of which there are 15, so if the difference came from the absence of hydrogen, then the amount of spheres would be 26. I therefore have no explanation of where the artist went astray in rendering his model, but it is clear that the molecular model doesn't depict the pigment that he claims.

This could also already be gleaned from the inclusion of 'Calcium' in the sculpture's title. Organocalcium compounds are very uncommon and so the inclusion of calcium in the name most likely means that this is a salt. The SO₃^1- sulphonate group in the molecule, shown in yellow with red, is very reactive  and needs to be ionically bonded to a positively charged atom, Ca²⁺ in this case, to be stable. The double positive charge on the calcium ion is paired with two single negative charges on the other compound, which means that there must be two of the previously shown molecule in the following configuration:

This is of course looks nothing like the molecule in Moulène's sculpture and anybody with knowledge of chemistry could have spotted the error merely from the first word of the title. 

I then noticed the following drawing on the cover of Keith Tyson's publication 'Molecular Compound No 4.':

Comparing this image with the VSEPR models at the beginning of this post, it should be clear that this drawing is not based on any existing molecule. Upon consulting the book, it turned out to contain no further references to reality and consist only of the fantastical imaginings of the artist, so I won't make any further comment on this publication.

I could list more examples of artists that have attempted to employ molecular models, but in short all of these sculptures I've encountered forgone scientific accuracy in some way.
The only one I know of that isn't necessarily wrong was a sculpture that simply used nothing but a commercially available molecular modelling kit. So while this was possibly accurate, it's artistic value was also negligible.

And the reason I've written all this is because I researched the subject while making the following model of a molecule called methyl mercaptan:

Methyl mercaptan, or CH₃SH, is one of the molecules that make farts smell. This model is made of a tennis ball, a black golf ball and four small roulette balls. These generic, store bought, balls are both the right colour and approximately the right size for a CPK-model for a molecular structure, as can be seen in this rendering taken from a molecular drawing program:

This is thus an indication that it's possible to have a novel approach to creating a molecular model without necessarily having to significantly compromise its scientific accuracy.

Some experiments

Towards the end of 2024 I performed some research on the chemical composition of a number of watercolour paints. In those experiments the mass fraction of the pigment was determined using UV-vis spectroscopy and thermogravimetric analysis in a number of paints with a quinacridone pigment. 

During that research I correlated some properties of the paint, such as sheen, to this mass fraction and the composition of the binder, which is gum Arabic. This in turn led me to make some predictions about their behaviour in water, which is the ultimate application of the paint.

In order to test these predictions, I devised a simple experiment where a small amount of dissolved watercolour paint was introduced with a brush to a small channel of water. Then it was merely a matter of observing the behaviour of the paint in the water.

Six watercolour paints with PV 19 pigment in water after 0, 15 and 60 minutes.

In the above image you can see the dissolution of six paints in water over time. From left to right they are arranged from low to high pigmentation. This also corresponds to a transition from large to small particles of gum Arabic.
As you can see, the smaller particles of gum Arabic, with higher pigmentation, generally dissolve faster and show more movement in the paint. It is however notable that the paint with the highest pigmentation and lowest particle size dissolves less fast than the other paints with high pigmentation. An explanation for this counter-intuitive observation is that the more tightly packed particles have less room for the water to enter and thus dissolve the paint.

Five Schminke Horadam watercolours in water after 0, 15 and 60 minutes.

I then repeated this experiment with five paints of the same brand, that I assume to posses a similar composition of paint. Their behaviour was indeed similar, while some small differences were still present. These can be explained by the differences in chemical composition of the various pigments, which will give different properties during the mixing and milling of the paint.

Winsor & Newton Professional (l) and Cotman (r) watercolour in water after 0, 15 and 60 minutes.

As a final experiment I tested if there was some validity to the observation that paints with a more matte appearance have larger gum Arabic particles and therefore dissolve more slowly. In the above image two paints from the same brand are tested. They are of different qualities, with the Cotman branded paint having less sheen than the Professional branded one. As it is clear to see, the professional branded paint, with higher sheen, did dissolve much more readily than the Cotman branded paint, which is an indication that the hypothesis might be correct. 

With these positive results, I then repeated these tests with a more common version of paper chromatography. The liquid phase in this instance is of course water.

From left to right; VG, RT, SCH, SCH', DS, DS', DS'' and KP

The first of these chromatography tests immediately produced some interesting results. The same paints are presented in the same order as in the first image. For the first three paints, we also see the same behaviour, where increased pigmentation and smaller particle size likewise give a further travel in the paper. It is then for the two highest pigmented paints that we see almost no travel at all. This was surprising so the experiment was repeated with the SCH and DS paints, to ensure that no kind of error was made while performing the experiment.
As this wasn't the case I can only assume that there is some kind of limit, whereafter the solubility of the paint in water is actually hindered by the paint particles being small enough to get 'stuck' in the fibres of the paper. So while smaller particles are generally are more easily brought into solution, there is probably some point where the paint particles are small enough to penetrate more deeply into the paper and then aren't removed as easily.

Winsor & Newton Cotman (l) and Professional (r) Permanent Rose

This is somewhat confirmed by the above experiment. Winsor & Newton paint was the slowest to dissolve in the first test and consequently they showed no travel at all in the paper chromatography experiment. This result is in line with the reasoning that larger particles of gum Arabic dissolve less quickly in water.

Schminke Horadam Mars Black, Mars Brown, Ochre, Lamp Black and Quinacridone Rose

A further test with the most soluble paint, Schminke Horadam, shows a near identical result as in the earlier tests. The explanations for this result are of course also similar to what we have already seen.

With the knowledge gained from these experiments in can therefore be said that paints with large particles of gum Arabic have generally lower pigmentation and dissolve less quickly in water. There is however a turning point when the paint is applied on paper, where smaller particles of gum Arabic attach more readily to the paper fibres and therefore adhere more strongly to the paper, hindering dissolution.

Dissolution Upon Contact With Water

When making some of the works from the γ-series, I noticed, or seemed to notice, that the watercolour was retained in the brush to a greater extent when using real sable hairs as opposed to synthetic materials. As this can influence the amount of control when has on the introduction of the watercolour to the droplet, I decided to test a number of different brushes.

For this experiment, a single batch was made of an unspecified amount of Royal Talens Rembrandt branded Quinacridone Violet (593) dissolved in 0,5 mL of distilled water.

Using a micropipette, a 5 µL droplet of distilled water was placed on Hahnemühle 290gms Agave watercolour paper.

Each brush was then dipped into the watercolour solution, rinsed two times by dipping it into distilled water and then placed into the droplet, as vertical as possible.

The action of adding the watercolour to the droplet was recorded by video. As the droplet had the tendency to move towards the brush, the first frame is recorded when the first movement of the droplet is observed, together with the frames 0,1 and 1 second after after this initial movement.

The following brushes were tested: Da Vinci Colineo, size 2/0; Da Vinci Fit Synthetics, size 0; Da Vinci Forte Synthetics, size 3/0; Escoda Perla Sintético, size 3/0; Gerstaecker, size 1; Winsor & Newton Synthetic Sable Round, size 00; Winsor & Newton Cotman 111 Round, size 00; Raphaël Martora Red Sable, size 3/0.

The experiment was repeated with Royal Talens Rembrandt branded Chromium Oxide Green (668). This colour consists of a simple inorganic pigment in the form of chromium(III) oxide, as opposed to the aromatic quinacridone pigment found in Quinacridone Violet.

This gave the following results:

Da Vinci Colineo, size 2/0

Da Vinci Fit Synthetics, size 0

Da Vinci Forte Synthetics, size 3/0

Escoda Perla Sintético, size 3/0

Gerstaecker, size 1

Winsor & Newton Synthetic Sable Round, size 00

Winsor & Newton Cotman 111 Round, size 00

Raphaël Martora Red Sable, size 3/0

An overview of all these tests gave the impression that the chromium oxide had more of a tendency to leave the brush than the quinacridone pigmented watercolour. The amounts also seemed to vary from brush to brush, with the additional remark that brushes of the same brand seemed to show similar diffusion.
Of these brushes the Escoda Perla Sintético brush had the least diffusion into the droplet. As this was also the most previously used brush, I wondered if this was perhaps related to the amount of use this particular brush has had. In order to find out, I bought an identical brush and redid the above test with both the new and the old brush:

Escoda Perla Sintético, size 3/0, used (top) and new (bottom)

In addition to the Quinacridone Violet and Chromium(III) oxide, I also used Royal Talens Permanent Lemon Yellow (254), which consist of bismuth vanadate.
There was very little difference between the new and the old brush. Perhaps the older brush had slightly more diffusion than the newer brush, but this can't be said with any certainty.

Another noteworthy observation is that the Quinacridone Violet didn't seem to dissolve into the droplet at all from the Gerstaecker branded brush.
Additionally, the only brush that contains the much-coveted real sable hair, the Raphaël Martora Red Sable, has a very different diffusion pattern from all the other brushes. The hairs of the brush spread far more easily than those in any of other, synthetic, brushers.

Related to this is the observation that the water in the droplet is very much attracted to the water present in the brush. This attraction is so strong that with a minimal amount of water, the droplet is almost 'sucking' the water out of the brush:

The International System of Units

My city, like many other cities, has a waste recycling point. This recycling point is funded by city taxes and thus every inhabitant of the city gets twelve 'free' visits a year. Of course, there is a limit to the amount of waste one can bring to the recycling point on each of these visits. 

On their website the city has posted some guidelines for a number of categories of waste. My favourite of these is 'Small Chemical Waste', of which one can bring a 'maximum 0.5 m³ per visit, including latex and paint'.
This never stops being funny to me, as my city apparently considers six thousands litres per year a perfectly normal amount of chemical waste for a regular household.

Benzene

The chemical benzene is apparently an attractive proposition for a number of artists.
Benzene has a unique spatial structure that makes it stand out from other molecules. It has been known since the 1800's that benzene consists of six carbon atoms and six hydrogen atoms. It was also known that carbon-based molecules are generally spatially arranged in connecting tetrahedrons. As this is impossible to achieve with an equal number of carbon and hydrogen atoms, it has been a long standing mystery on how these atoms were arranged in the molecule.
The beginning of the solution was offered in 1865 by August Kekulé, who proposed a geometrically flat hexagonal 'ring' structure with alternating 'double bonds', which he visualised in the following manner:

This is the actual model Kekulé built to demonstrate the structure. It is now in the collection of the University Museum in Ghent, Belgium, where Kekulé was living at the time.
In this model the black balls represent carbon atoms, the white balls are hydrogen atoms and the connecting metal rods are single and double bonds. Such bonds are connections between atoms created through both atoms 'sharing' a pair of electrons. In a double bond two pairs of electrons are thus shared between two adjacent atoms.
After the 19th century, quantum mechanics and molecular orbital theory have further refined this view. Yet the general principle of benzene consisting of six carbon atoms in a planar hexagonal shape still stands, with the six hydrogen atoms arranged like 'antennas' at opposite ends of, and in the same plane as, the hexagon. As such, it is usually graphically depicted in one of the following ways:

It's obvious that artist Monira Al Qadiri had these (simplified) graphical structures in mind when she started on her work 'Benzene' in 2022. This work consists of a series of sculptures where, according to her, 'the scientific geometry of benzene's chemical compounds are rendered into glass sculptures, in order to highlight the grip that this perfumed molecule has on our lives.' While most of these structures are straightforward translations of the above shown schematics into three-dimensional glass shapes, one of them caught my attention:

To any chemist it's immediately obvious that this is completely impossible and has nothing to do with any kind of reality. Al Qadiri's claim to 'the scientific geometry' is thus hardly scientific.
To understand why this is so you need a bit of technical understanding about delocalized π-electrons in the structure of benzene and where possible lone pairs of electrons would go if their hydrogen atoms were displaced. Since providing such understanding isn't really attainable within the scope of this blogpost, let me just say that Al Qadiri's sculpture is a bit like stating that this is what a functional bicycle looks like:

Al Qadiri further places emphasis on the smell of benzene. She says that benzene is 'a colourless and highly flammable liquid with a sweet smell, it is partially responsible for the aroma around petrol stations, and is thus classified as an ‘aromatic hydrocarbon.’ ' It is in this manner that she makes the connection between benzene and the petrochemical industry. While benzene is (non-exclusively) extracted from crude oil, the connection she makes with petrol stations is partially a false one. Benzene is a minor part of gasoline, of only approximately 1% by volume. It thus doesn't contribute greatly to any particular core property of gasoline, least of all it's flammability. This flammability is much more influenced by short-chain alkanes like butane and hexane, which have far lower boiling points and oxidize much more rapidly. In fact, a mixture of benzene and benzene-like molecules called BTEX is sometimes added to gasoline to reduce its combustibility.
The second part of her statement, where she links the smell of benzene to its classification as an 'aromatic hydrocarbon' misunderstands cause and effect. It is true that in 1855 August Wilhelm Hoffman gave the classification of 'aromatic acids' to a number of compounds, even if not all them had  a distinctive smell. We now know the core component of those 'aromatic acids' was the presence of a benzene-like structure and the 'aromatic' moniker has thus stuck for those kind of molecules. Their properties and uses vary wildly, however. Besides benzene, photographic developer is also aromatic and so are the basic building blocks of DNA. Al Qadiri's observation is thus far removed from an explanation of any of benzene's properties or 'the grip it has on our lives'.

Less poetically interpretative, but equally misinformed, is a much earlier example by the hand of Bernar Venet. At the time known for his appropriated 'scientific' drawings, Venet made the following 'drawing' in 1966:

The text, in French, describes the 'importance of Kekulé's formula'. According to Venet this 'formula allowed us to interpret the hydrogenation of benzene to cyclohexane and the chlorination to benzene hexachloride.'
I'm not exactly sure what he's attempting to express here. Hydrogenation of alkenes to alkanes using platinum catalysts was first published in 1874, some ten years after Kekulé's discovery, and I'm not entirely sure this process would work on the more stable structure of benzene. How it helps us 'interpret' this process is thus unclear to me, as it implies that the discovery of the process came after the explanation of that process.
Furthermore, the process of 'chlorination' generally refers to simple addition of chlorine to a molecule. In this case you would thus end up with hexachlorocylcohexane, not hexachlorobenzene as is claimed in Venet's text. But it is possible to make hexachlorobenzene from benzene with a substitution reaction, so lets just assume Venet meant this instead. In that case he describes the structural formula of 'hexachlorobenzene' as C₆H₁₂Cl₆. This formula is simply impossible. A carbon atom can only be connected to four other atoms at the same time. In a ring structure, two of those possibilities are already taken up by the neighbouring carbon atoms, which leaves us with a total of 12 'free' spaces. As we have 6 chlorine atoms and 12 hydrogen atoms in Venet's proposed formula, we apparently need to fit 18 atoms into the 12 available possibilities. The correct formula would thus be C₆H₆Cl₆ for hexachlorocyclohexane or C₆Cl₆ for hexachlorobenzene.
Furthermore, in order to show the formation of benzene, Venet uses the so-called trimerisation of alkyne as his example. This is an unusual choice from a chemical point of view. Firstly because this process was first described in 1866, one year after Kekulé published his formula. And secondly because trimerisation is a very difficult reaction to perform. It has a very high activation energy, thus requiring high temperatures of >800 ºC, and even then the end result isn't pure benzene but a mixture of different products. Therefore this reaction was far from efficient, or common, until a different process was developed in the late 1940's that involves the use of catalysts, which made alkyne trimerisation a viable reaction in routine synthesis work.
Thus while I generally enjoy the drawings of Venet for their stylized simplicity, it's best to not actually read the text that's contained in them.

Richard Venlet is a third artist I've encountered who has an interest in benzene and it's structure. He published a booklet with the title Kekulé in 2011. Its starting point was the anecdote of August Kekulé's first insight into the structure, which took place while he was living in Ghent, Belgium. 

Venlet presents no claims to scientific knowledge and his little booklet seems to be nothing more than a happenstance that reflects his interest in hexagonal shapes, like the ones he used for a series of floor panels created for Maniera two years prior:

As is clear to see, in the booklet Venlet presents a repeating pattern of hexagons, as he would show in his exhibitions. The reference to Kekule and his structure of benzene is thus a pure formal one. Nevertheless, it must be pointed out that the structure he presents is chemically impossible. The flat structure of such a system, like in the well-known graphene, is only possible through the 'double bonds' present in benzene, or rather its delocalized π-system. Such a system is usually represented by the addition of extra lines in certain places, which are missing in Venlet's drawing. Although seemingly a small difference, this has great consequences for the spatial arrangement of such a molecule, which would put an impossibly large strain on the system that Venlet represents. The following illustration hopefully gives a sense of the factual differences that are left out of such simplified illustrations:

It should be easy to see that a so-called saturated system that Venlet has drawn is far more crowded and therefore possesses very little room to move and wiggle, something all atoms want to do. While it might be possible on a smaller scale like the above illustration, a large field like the one presented in Venlet's booklet will in reality simply fall apart and find a different conformation.

In conclusion I should once again state that although I have never expected otherwise and can occasionally enjoy the fantasy-rich interpretations of artists, it's nevertheless a good idea to presume that an artist's factual understanding of the natural sciences is negligible. When I asked as a chemist I know why he enjoyed working with artists, he simply said 'it's so nice to see people who are unburdened by knowledge'.

Testing, Testing.

Recently I wrote about some watercolours I've made. Since then I've found some scientific literature on the subject, after discovering that the 'coffee ring effect' is the scientific name of a ring shaped deposit found after a drop of liquid has dried. It's a relatively new field of study, with major research only being done since the late 1990's. This literature does confirm my basic assumption of the movement of the paint particles, which is explained by capillary flow. The literature also shows that there are many competing phenomena and variables at play, which are difficult to measure and analyse. Many of the papers I found focus on variables like temperature, relative humidity and electromagnetic influences, most of which effect the rate of evaporation.

I've done some experiments to test the influence of some of these parameters on the appearance of my own drops of watercolour, with some notable results.

First I tried to measure the influence of temperature. The results of this were mostly inconclusive. To test the influence of temperature, I uniformly applied the droplets at three different temperatures, to see if their appearance would differ after drying. The expected result from some of the literature would be that a higher temperature creates a more even distribution throughout the drying droplet. Various mechanisms have been suggested on how this works, including a greater evaporation at the contact surface with the air, which causes greater flow inside the droplet, as well as a 'surface capture' effect of particles at the contact surface.
In the rudimentary testing I have done I however didn't notice any significant effects of temperature on how uniformly the paint spread through the drying droplet:

Three drops dried at different temperatures

In this image there are three droplets of about 2 mm in diameter, made with Winsor and Newton's Payne's grey watercolour paint. The first was made on a substrate that's cooled below 0ºC, the middle was made at room temperature and the last one was heated after application in an oven to about 70ºC. It's clear that there is little significant variation between these three droplets, thereby giving indication that temperature, at least on this scale and with these materials, is not a significant contributing factor for the distribution of the pigments in the drying droplet.
However, the influence of temperature might be dependent on the exact chemical composition of the pigments, in combination with corresponding changes in the binders used. The following image consists of the results of the same experiment, showing Daniel Smith's Hematite Genuine watercolour paint, in duplicate, at <0ºC, room temperature and ~70ºC, respectively.

Two sets of three drops dried at different temperatures

What one can observe here is greater ring formation with a cooled substrate and more concentration at the center at elevated temperatures. So much so that the ring where the pigment is deposited is not even found at the outer edge of the droplet, which is something I have not observed in other situations. This behaviour is also the exact opposite of what the literature would have us expect.

When examining the literature, it must also be noted that most of the literature on the coffee ring effect seeks to eliminate it, because in an analytical or manufacturing context its existence is commonly detrimental to achieving uniform depositions or measurements. Relatively little literature thus exists on controlling the formation of the ring itself, and as far as I can tell, all research is done on colloids that are mixed prior to droplet formation. Little to no research has been done on the effects of introducing a colloid to an existing droplet. Yet I've found indications that for our purposes this provides a lot of control on the exact formation of the coffee ring, as can be seen in the following image:

Four different ways of introducing the paint

From left to right, this is a simple droplet of a diluted suspension of Winsor and Newton Payne's grey watercolour, a water droplet to which a diluted suspension was added at the centre point of the droplet after droplet formation, a water droplet to which a diluted suspension was added at the right edge of the droplet after droplet formation and a water droplet to which a near-saturated suspension was added at the right edge of the droplet after droplet formation.
As you can see, the two leftmost droplets dried nearly identical, even if their method of application was very different. For the third droplet from the left, paint was added later at an angle on the right edge with the paper, and this saw most of the pigment end up around the full perimeter of the droplet. This process was repeated with a higher concentration of pigment in the last droplet and while this contained far more pigment than the other three droplets, still most of it stayed at the perimeter of the droplet, with even more seemingly remaining at the initial point of introduction.

My explanation for this is that a similar outward pushing effect is at work here, inhibiting the possibility for pigments to enter the centre of the droplet through gravity or other forces.
It must however be also noted that in some degree this is dependent on the exact shape of the droplet and again the composition of the paint.

Three different ways of introducing the paint

In this image we have a droplet with a homogenous solution of Daniel Smith's Venetian Red water colour paint, followed by a saturated solution of the same paint added at the right edge of a droplet of water and ultimately a heavily diluted solution added at the right edge of a droplet of water. They each have their distinctive appearances, which differ subtly from the previous experiment with Payne's grey, most notably with the later introduction of a saturated solution. This produced a light centre with a thick edge in the previous experiment, while it created a mostly even spread with a thin edge in the latter example.

Even though it's difficult to observe this behaviour in real time and at actual scale, I believe the observations from the previous two figures is related to the behaviour of the pigment at the droplet's contact surface with air. I did a test where I placed a small saturated spot of Payne's grey watercolour on a piece of paper, let it dry, and then added a water droplet, without physically disturbing the spot of paint. What I found after this droplet had dried is that the paint had spread uniformly throughout the droplet, with a clear coffee ring effect present. There thus is a tendency for the paint to be distributed inside the droplet if it gets far enough inside. 

Adding water to a dried spot of paint

Generally speaking, predicting the exact behaviour of the interaction of a fluid and a colloid is complex and very difficult, as can be seen in the following example:

Introducing two paints into a single droplet

In this image two different watercolour paints are added to a single droplet. The droplet at the top was a diluted solution of Daniel Smith's Quinacridone Gold water colour paint, to which a saturated solution of Daniel Smith's Quinacridone Red was added on the right side at an angle. The droplet at the bottom was pure water, to which Quinacridone Gold was first added at the top and then Quinacridone Red was added on the right side at an angle. As is clearly visible, the latter process resulted in a nearly homogenous mixture, while the first gave a degree of separation between in the colours in the dried droplet.
However, I then repeated this experiment using Daniel Smith's Quinacridone Gold and Winsor & Newton's Payne's grey.

Introducing two paints into a single droplet

Here the same procedure was followed, with the Payne's Gray being added first, followed by Quincridone Gold on the right side at an angle. The way the paints mixed was the opposite of what I observed in the previous experiment. On this occasion the Quinacridone Gold mixed better with the droplet of diluted water colour, while the two paints stayed separated when added in sequence to a droplet of pure water. At the present time I have no simple explanation for this seeming contradiction in behaviour.

Lastly I want to note another characteristic I hadn't considered up until this point, which is the influence of magnetic effects on the droplets. Naturally electromagnetic effects are strong if there are ferromagnetic pigments present in the paint. Especially in the case of paints that contain a mixture of magnetic and non-magnetic pigments, introducing a magnetic field during the drying process produces interesting effects that can be easily controlled with the presence of any magnetic field. 

In conclusion, about a month has past since the previous post and I have still made some new observations about the behaviour of the watercolour paint inside a droplet. Some of these observations seemingly contradict the explanations found in current scientific literature, while others provide a possibility for new methods that are hitherto unexplored.