Schulze's scotophorus mixture

Photography’s first formula does not contain a number. Johann Heinrich Schulze was trying to make Balduin’s luminous stone, which is chalk dissolved in nitric acid; the acid nearest to hand was the kind an assay office keeps for parting gold from silver, which carries dissolved silver; and the paste he made with it darkened on the side facing the window. He spent the next two years proving that the agent was light and not heat, wrote words into the sediment with paper stencils, and published the whole of it in 1727 under a title that reads as a joke against himself: he had gone looking for a bearer of light and found a bearer of darkness.

Everything below is his, and the composition is stated by operation from beginning to end.

This formula's source publishes no quantities. The acid is "imbued with a quite moderate quantity of silver particles, namely as much as is needed to prepare it so that it be fit for the parting of gold from silver"; the chalk is imbued with that acid until the portion is saturated and stands as a thick paste; the water is "a portion of water" poured on to check the effervescence; and the strongest variant is aqua fortis "imbued with as great an abundance of silver as it is able to dissolve". No mass, volume, percentage or ratio appears in the memoir or in any later printing of it.

The composition is fixed by saturation — the solvent is charged until it will take no more. By two saturations in series, and the second one is the composition. The silver is fixed against the acid — either at the assayer's charge that makes parting acid fit for parting, or, in the strongest version, at the limit of what the acid will dissolve, which is a definite concentration reached by an operation rather than by weighing. The chalk is then fixed against the liquid: acid is worked into the powder until it will take no more and the mass stands as a thick paste, which is a repeatable end point that anybody with a dish and a spatula reaches the same way. What is left undetermined is the ratio of silver to chalk, and therefore the sensitivity, which is why Schulze can only report that more silver gave a far stronger colour and cannot say how much more. A modern reader can reproduce the operation and cannot reproduce the mixture.

The amounts below are the source's own words, not measurements, and the table is not a recipe. 6 documents were read for a published quantity and none states one; they are listed in this page's sources.

Ingredient	As the source states it	Form the source specifies
Nitric acid	“aqua fortis, in no stated amount”	Latin "aqua fortis", German "Scheidewasser", the parting acid of the assay office; and, in the two control experiments that failed, fuming "spiritus nitri" tempered with much water and aqua fortis "as sold in the pharmacies"
Silver	“as much as is needed to prepare the acid so that it be fit for the parting of gold from silver”	Metallic silver, dissolved in the acid rather than added as a salt. In the strongest variant Schulze reports, the acid carries "as great an abundance of silver as it is able to dissolve"
Calcium carbonate	“imbued with the silvered acid until the portion is saturated and stands as a thick paste”	Latin "creta", powdered chalk. Schulze substituted "cerussa", white lead, in several experiments with almost the same success, and names burnt hartshorn and white magnesia as likely substitutes he did not try
Water	“a portion of water, poured on to check the effervescence, and again to temper the silver solution”	Water appears twice and is measured neither time. It is poured over the chalk and acid to stop the mass boiling up and dissolving, and in the deliberate repetition the silver solution is "tempered" with water before the chalk is imbued with it. The dilution is therefore part of the composition and is not stated.

☠Level C: this page describes an experiment and gives no procedure

The acid here is nitric acid at parting-acid strength, which is Level C in this course’s chemical encyclopaedia, poured onto a carbonate so that it effervesces. Schulze did it at an open window in 1725 and had to pour water on to stop it climbing out of the vessel. No procedure for making this mixture appears on this page, and the quantities that would let a reader try are not withheld — they were never published.

The route to the same chemistry that the course does teach is salted paper, whose sensitiser and salting solution carry every quantity and sit at Level B. The nearest thing to this preparation with a number attached is Wedgwood and Davy’s silver nitrate on leather of 1802 — seventy-five years later, and the first photographic formula that says how much.

The Part I lesson owns the history and the argument about who invented photography. This page owns the memoir as a formula: what Schulze actually put in the phial, how he fixed the composition without measuring it, and what a reader can and cannot take from six pages of Latin.

Purpose

To make a white solid darken in sunlight and not in heat, and then to write on it. Schulze’s stated purpose was neither: he wanted Balduin’s phosphorus, a luminous stone, and this is what he got instead. The purpose the mixture actually serves — his, once he had it, and the course’s now — is demonstration. It shows that a silver salt is changed by light alone, that the change is confined to where the light falls, and that a shadow cast on it is recorded.

That is the whole of photography’s premise, seventy-five years before anybody tried to keep a picture made from it.

Recommended uses

None practical. This is a historical-study entry and the honest answer to “what is it for” is reading, dating and understanding.

Three uses it serves in a modern course.

• Reading the 1727 text. Anybody who meets scotophorus, creta saturata, aqua fortis or atro-rubentem et in coeruleum vergentem in a history has somewhere to look them up, and can see what the words were attached to.
• Understanding what a formula was before formulas had numbers. The whole of this page’s difficulty is that its source states composition by operation. That is not carelessness and it is not a lost manuscript: it is how a working chemist of 1727 specified a preparation, and it is worth meeting once before reading Kodak.
• Seeing the permanence problem at its origin. The mixture has no fixer and cannot have one. One shake of the phial erased Schulze’s writing and made the material ready for the next exposure, which he reports as a convenience. It is the same defect that stopped Wedgwood and Davy in 1802 and was not solved until Herschel in 1819 and its application in 1839.

When another formula is preferable

Always, for anything a reader intends to make.

For the same silver chemistry with published quantities: the salted paper sensitiser over Reilly’s plain salting solution, fixed in a plain thiosulfate bath. Level B, every figure sourced, and a picture at the end of it.

For the oldest formula that a reader can actually mix: Wedgwood and Davy’s silver nitrate on leather, 1802. One part of silver nitrate to about ten of water is one sentence, and one sentence is enough — which is exactly the difference between that page and this one.

For a photogram without a camera or a darkroom: Talbot’s photogenic drawing paper, and the photogenic drawing atlas entry with it.

For the other end of Part I’s chronology: Daguerre’s 1839 process outline, which is the same problem in the other direction — a source that publishes proportions and vessels where the schema wants numbers, and which this course scaled and said so.

Mixing

Schulze mixes twice, and the second time is the formula.

The accident. He took a portion of parting acid that already had silver in it, and used it to imbue chalk, because that is what Balduin’s process requires. The chalk effervesced, the mass saturated, and the surface exposed to the window turned dark red inclining to violet within minutes while the shaded part of the dish showed the colour “not in the least”. To get the thick paste into a round medicine phial he poured on more aqua fortis; it boiled up and began to dissolve the chalk, so he poured a portion of water over it to check that. Then he set the phial in the sun.

The deliberate repetition, which is the version this page records, is one sentence: argenti portionem aqua forti dissolvo, dissolutum aqua tempero, et cretam, sicut antea, imbuo — I dissolve a portion of silver in aqua fortis, I temper the solution with water, and I imbue the chalk as before. Four operations, four unmeasured amounts, and the same phenomena as before but with the colour coming out “far more manifestly”, because there was more silver in the liquor.

!What fixes the composition, since the numbers do not

Two saturations in series, and only the second of them is a saturation in the strict sense.

The silver against the acid. In the accident, the charge is the assayer’s: enough silver to make aqua fortis fit for parting gold from silver, which is a real and reproducible specification inside a trade that Schulze assumes his reader knows. In the strongest version he reports, the charge is the solubility limit — aqua fortis “imbued with as great an abundance of silver as it is able to dissolve” — and that is a definite concentration, arrived at by an operation instead of a balance. Anybody can hit it; nobody can write it down without knowing the acid’s strength, which Schulze never gives.

The chalk against the liquid. Acid is worked into the powder until the portion is saturata and the mass stands as a spissum magma, a thick paste. That end point is reached by feel, and it is repeatable by feel, which is how every worker of the period specified a paste.

What neither operation fixes is the ratio of silver to chalk, and that is the sensitivity. It is why Schulze can say that more silver gave a stronger colour and cannot say how much more, and why this page prints his words and no arithmetic beside them.

The order matters and is his. The silver goes into the acid; the solution is tempered with water; the chalk is imbued last. Reversing the first two would be pouring acid into a metal vessel of water, and putting the chalk in before the water would take the effervescence he had already had trouble with and make it worse.

Behaviour

The mixture is a print-out material with no development and no fixing stage, and everything it does follows from that.

It darkens where the light falls and nowhere else. The part of the dish the rays did not reach showed the colour “not in the least”; the back of the phial, which no ray reached, stayed white; a thread laid down the front of the glass left a clean pale line after hours in the hottest sun, and so did a horse hair, a human hair and the finest silver thread Schulze could get.

It does not respond to heat. Held to a hearth fire, unexposed side towards the flame, until the glass was almost too hot to hold, it did not change colour at all. This is the control that makes the memoir an experiment rather than an anecdote, and it is the first time anybody separated the chemical action of light from the action of heat.

It responds to reflected light, more slowly. A plane mirror throwing sun into the phial worked as well as direct sun. A house opposite the window, freshly limewashed and “dazzlingly white”, threw enough morning light across the room to darken a phial left there overnight. Schulze draws the practical conclusion himself: anybody repeating the experiment carefully must make sure there is no reflecting body behind the vessel, or the shadow side will not stay clean.

It is erased by stirring. Shaking the sediment and the supernatant liquid together destroyed the image entirely and left the material ready for the next exposure. Schulze used this all afternoon. There is no way to keep a picture made in it, and no chemistry in 1727 that could have provided one.

It keeps, unexposed, in the dark. The second phial, filled from the same batch and put in a dark place, kept the same whitish colour every time he looked, with no trace of change in any part of it. The saturated chalk he had dried and put aside changed colour promptly whenever it was brought out into the open sun.

Image characteristics

There is no image in the sense the rest of this formulary uses the word: no latent image, no development, no support and no object. What there is is a stain in the top of a settled powder, in the shape of whatever masked it.

Colour. Dark red inclining to violet on the open paste, blackish red verging on blue in the phial. Schulze’s atro-rubentem et in coeruleum vergentem is the phrase Eder quotes and every history after him repeats. It is a plausible description of finely divided photolytic silver in a white matrix, which is the same colour question the salt print raises for a reader who has ever wondered why a print-out image is not black.

Resolution, of a kind. Names and whole sentences, cut from paper with a sharp knife and waxed to the glass, came out “so accurately and distinctly” in the chalky sediment that visitors who did not know the experiment supposed some trick. A silver thread’s shadow resolved as a clean line. That is better than the material has any right to give, and it is because the mask was in contact with the glass and the sediment lay against it — the same geometry as a contact print.

Permanence: none. The colour lasted some days in the dried paste, “until it was consumed by further experiments”. In the phial it lasted until somebody moved it.

The mechanism

Four steps, and Schulze established the second and fourth of them experimentally without being able to name any of the substances.

The silver dissolves. Metallic silver in nitric acid gives silver nitrate, and the nitrogen monoxide that comes off browns to the dioxide in air — the ruddy vapour that every eighteenth-century account of a metal dissolved in acid of nitre remarks on, Scheele’s included.

3 Ag + 4 HNO₃ → 3 AgNO₃ + NO + 2 H₂O

Making the silvered acid, which in the accidental version had already been made by the assayer

The chalk is attacked, which is what Balduin’s process is for. The effervescence Schulze had to check with water is carbon dioxide, and the calcium nitrate left behind is the luminous stone he was trying to make.

CaCO₃ + 2 HNO₃ → Ca²⁺ + 2 NO₃⁻ + CO₂ + H₂O

The reaction Schulze intended, written for the solution

A silver salt is precipitated onto the chalk. Silver nitrate meeting the excess of undissolved carbonate throws down silver carbonate, which is yellow, insoluble and light-sensitive.

2 AgNO₃ + CaCO₃ → Ag₂CO₃ + Ca²⁺ + 2 NO₃⁻

The sensitive salt, formed in the vessel from two things neither of which is sensitive alone

Light reduces it to metal.

2 Ag₂CO₃ + light → 4 Ag + 2 CO₂ + O₂

Net photolysis of silver carbonate, written as the overall change

⚗What is actually darkening, and how sure anyone is

The equations above are the course’s reading of Schulze’s observations, not Schulze’s chemistry. He had no way to write any of them: the memoir predates Lavoisier by two generations, and its author’s best explanation is that the effect of the sun’s light and heat is different from what one may expect of a kitchen fire.

Ware names silver carbonate as the sensitive body, formed adventitiously and decomposing photochemically, and that is the cleanest account of why chalk and a silver solution together are sensitive when neither is sensitive alone. Eder read the mixture as silver chloride and carbonate in silver nitrate against a white ground, and the chloride is not a stray guess, though the reason for it is the course’s and not Eder’s: on the assayer’s practice, a little silver is added to parting acid to bring chloride down as silver chloride, so an acid straight from that bottle may carry silver chloride in suspension — and silver chloride is more light-sensitive than silver carbonate. Nobody has assayed the residue in Schulze’s phial and nobody can.

Schulze himself narrowed it further than either. He exposed the silver solution, diluted with water and with no chalk at all, in an open glass, and the blackish colour appeared in the liquid just the same. So the chalk is not necessary to the darkening; it makes it visible and probably makes it faster. And photolysis of a silver salt is the reduction of silver ion to metal, with the counter-ion oxidised — the same net change the silver halide pages set out, and the reason Scheele could prove fifty years later that the black substance is silver.

The two null experiments are the load-bearing evidence and they belong here rather than in the history. Fuming spirit of nitre, tempered with much water so that it would not dissolve the chalk outright, wetted the chalk and produced no notable change of colour even in the brightest sun. Aqua fortis as sold in the pharmacies did the same nothing. Only the acid with silver in it worked. That is a controlled experiment identifying the active element, run in 1725, and it is the reason this entry exists.

Function of every ingredient

Nitric acid — aqua fortis, Scheidewasser, the parting acid. Its job is to hold the silver in solution, and its second job, which Schulze wanted and the photography did not need, is to attack the chalk and make calcium nitrate for Balduin’s luminous stone. It is not the sensitive body, and Schulze proved that better than anyone proved anything else in the memoir: he repeated the whole preparation with fuming nitric acid diluted with much water, and again with ordinary pharmacy aqua fortis, and neither darkened. Strength is unstated throughout. Period parting acid was strong, and the effervescence that nearly climbed out of the vessel says so; but “aqua fortis” is a trade name and not a concentration, and this page will not turn it into one. More acid than the chalk can neutralise dissolves the chalk, which is exactly what began to happen when he poured more on to make the paste pourable, and is why the water goes in.

Silver — the whole of the sensitivity. Dissolved as the metal, not added as a salt, which is why this page lists silver and not silver nitrate as the ingredient even though silver nitrate is what exists in the phial. Every positive result in the memoir has it and every negative result lacks it. Schulze also gives the dose-response, and gives it in the only terms he has: the colour came out far more manifestly when a greater abundance of silver particles was in the liquor, and with the acid charged to its limit even the parts of the glass no direct ray reached took a distinct blackness from reflected light alone. That is a stated relationship between concentration and speed, published in 1727, with no number on either axis — which is as good a one-line summary of this entry’s problem as the course can offer.

Calcium carbonate — chalk, and an accident twice over. It is in the formula only because Balduin’s phosphorus is made from it, and Schulze says so plainly: he used chalk powder by chance, since his intention had been to prepare Balduin’s phosphorus, and he thinks it makes little difference if somebody prefers another white body — burnt hartshorn, white magnesia, and the like. He tried white lead himself, with almost the same success, and rejected it for handling reasons rather than chemical ones: it stuck too firmly to the sides of the glass, sank too heavily by its own weight, and after standing was harder to mix back into the liquid, which has to be possible if the colour once taken up is to be removed and the material used again.

So the chalk does two things and only knew about one of them. Deliberately, it is a white ground, so that a darkening can be seen at all — the same job the paper does in a salt print. Accidentally, it is the carbonate that takes silver out of solution as an insoluble, light-sensitive salt spread over a large surface, which is very probably why the paste is quicker than the clear solution Schulze also exposed. Neither he nor anybody until Ware could have said the second half.

Water is not an encyclopaedia ingredient here but it is part of the composition and it is unmeasured twice: poured over the acid and chalk to stop the effervescence, and used to temper the silver solution before the chalk is imbued in the deliberate repetition. Its amount changes the strength of everything else on the page.

Interactions

Acid against carbonate. The pair is self-limiting only if the chalk is in excess. Schulze’s paste is chalk in excess — that is what saturata means here — and the moment he added more acid to make it pourable the balance tipped, the mass effervesced hard and the chalk began to dissolve. Water was his correction, and it works because it slows the reaction without removing any of the acid.

Silver against chloride, on the course’s reading of why the acid was silvered. No source read for this page says why parting acid carries silver; the assayer’s reason is that silver is added to bring chloride down as silver chloride before an assay. If that is the reason, any chloride in Schulze’s acid was already silver chloride, which is more light-sensitive than silver carbonate — which is an argument for Eder’s reading of the mixture over Ware’s, and is offered as an argument and not as a finding.

Silver against light and against a reflecting wall. The interaction Schulze warns his reader about is optical rather than chemical: put a white or polished body behind the vessel and the shadow side darkens too. He found this out by leaving a phial at a window opposite a freshly limewashed house.

Variants

Schulze publishes four himself, all in the same six pages, and none of them with a quantity.

• The accidental version, made with parting acid straight from the bottle. Slowest, and the one that started everything.
• The deliberate version, silver dissolved in aqua fortis and the solution tempered with water. Stronger, because more silver.
• The saturated version — aqua fortis carrying as much silver as it can dissolve. Fast enough that reflected light alone blackened the shaded side of the glass. This is the only place in the memoir where the composition is fixed by a limit rather than by a judgement, and it is the version a modern reader could most nearly reproduce.
• The chalk-free version: the same silver solution, diluted with water, exposed in an open glass with no chalk at all. It darkens. This is a control, not a working variant, and it is what tells us the chalk is a ground rather than the sensitive body.

He also names three substitutes for the chalk — white lead, which he used; and burnt hartshorn and white magnesia, which he did not — and reports that white lead behaved almost the same and handled worse.

Later relatives, in this formulary. The first descendant with a number in it is Wedgwood and Davy, 1802: one part of silver nitrate to about ten of water, which is a ratio, on a support, still with no fixer. The first with a fixer is Talbot’s photogenic drawing paper of 1839. This entry has neither, and is the reason both of those look like progress.

Safety

Nitric acid is the whole of the hazard and it is Level C. The encyclopaedia entry carries the classification, the exposure limits and the handling rules, and none of them are restated here, because this page is not asking anybody to handle it.

Three period details worth naming, since they are in the source and a reader may be tempted.

• The effervescence is violent and it is not optional. Schulze had a thick paste climb his vessel hard enough that he had to pour water on to stop it. Strong nitric acid on a carbonate in an open dish, indoors, at a window, is a thing this course would not do.
• Water into acid is the wrong direction. Schulze pours water onto an acid mixture, and the nitric acid page carries the instruction that is printed in capitals on the ILO-WHO card: never pour water into this substance. His mixture was already dilute and already reacting, which is not the same case as a bottle of concentrated acid, but the general instruction is the one a reader should carry away.
• Silver nitrate stains skin permanently, which the silver nitrate page covers and the eighteenth century did not consider a hazard.

There is nothing here that could not be seen, more safely and with a better result, in a salted paper exposure.

Storage

The source publishes no keeping figures, and there is no honest way to derive them, so this entry carries none. What it does publish is two observations about keeping, and they are better than a figure.

Unexposed, in the dark, it keeps indefinitely as far as Schulze could tell. A second phial filled from the same batch and set in a dark place retained the same whitish colour every time he returned to it and showed no trace of change in any part.

Dried, it stays sensitive. He dried a portion of the saturated chalk and kept it, and it changed colour promptly whenever it was set out in the open sun. This is the closest thing in the memoir to a storable sensitised material, and it is where the idea of a coated, dried, kept sensitive surface begins.

Exposed, it keeps nothing. Days at most in the dried paste, and in the phial only until it is moved.

Incompatibilities

Any acid-sensitive vessel or surface. Schulze works in glass and a glazed dish, which is right.

Anything that reflects. Covered under Interactions, and it is the incompatibility that actually spoils the experiment.

Organic matter, with the acid. Nitric acid at parting strength on paper, cloth, wood or skin is a hazard the encyclopaedia entry sets out. The paper stencils in the memoir are waxed to the outside of the glass and never touch the mixture.

Chloride, in the sense that it changes what you have made. Adding a chloride to this mixture makes silver chloride, which is a different and faster material — and, on the reading above, may already be part of what Schulze had.

Waste

Nothing here is prepared, so nothing here is disposed of. For completeness, since a reader may meet the residue in another context: the mixture is a silver salt in an excess of chalk, under a nitrate solution. Silver is recovered rather than poured away, by the routes the silver and silver nitrate entries give; the remaining calcium nitrate liquor is a fertiliser salt and a strong oxidiser, and is not a drain disposal on either count without checking local rules.

Troubleshooting

The memoir is unusually rich in failures, because Schulze published his controls. Each of these is his.

Nothing darkens at all. The acid has no silver in it. He got this twice on purpose — with fuming spirit of nitre tempered with much water, and with aqua fortis as sold in the pharmacies — and it is how he identified the active element.

The shaded side darkens too. Something behind the vessel is reflecting. A mirror does it deliberately; a limewashed wall across the street does it accidentally; a strongly silvered acid does it so readily that even indirect light suffices.

The chalk dissolves instead of thickening. Too much acid for the chalk present. Schulze’s own correction is to pour on water.

The image disappears. Something moved the vessel. This is not a fault and cannot be prevented; Schulze treats it as the way the material is reset.

The white solid cakes on the glass and will not remix. That is white lead, not chalk. His reason for going back to chalk is entirely this.

Nothing happens under the burning glass except that the glass gets hot. The focus is exactly on the vessel. His instruction is to fall a little short of it.

Experiments

Everything in this section is Schulze’s and is offered as reading, not as a procedure. The course does not prepare this mixture, and every experiment below can be run on salted paper at Level B with a better result.

• The heat control. Turn the unexposed side of the vessel to a fire and warm it until it is almost too hot to hold. Nothing changes colour. Running the control on the unexposed side is the detail that makes it airtight, and it is Schulze’s.
• The thread. Shake the mixture uniform, hang a fine thread down the front of the vessel, leave it in the strongest sun for some hours, and lift the thread off. Repeat with horse hair, human hair and the finest silver wire available. This is a resolution test and it is 1725.
• The stencil. Write names or whole sentences on paper, cut the inked parts out with a sharp knife, and wax the perforated paper to the glass. The sun writes them into the sediment. Shake to erase, and do it again.
• The mirror and the wall. Reflect sunlight into the vessel from a plane mirror; then leave it overnight facing a white sunlit wall. Both work, the second more slowly. Then place a white body behind the vessel and watch the experiment spoil.
• The burning glass. Collect the sun with a convex lens, take care to fall a little short of the focus, and the darkening is all but instantaneous.
• The null runs. Repeat the whole thing with acid that has no silver in it. Nothing.

🧪The one this course would set instead

Make a salted paper sheet, expose a thread and a paper stencil on it in the sun, and fix half of it in a plain thiosulfate bath. Leave the unfixed half in daylight for a week.

You will have reproduced every observation on this page, at Level B, with every quantity written down — and the unfixed half will teach you, in seven days, the thing Schulze could only demonstrate by shaking a bottle.