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Pure Silver

Chemical encyclopaedia

A photographic formula is a list of chemicals, and a photographer who knows what each one is can read any formula, not just the ones they have been given. This encyclopaedia has one page per substance the course handles or discusses. Each gives the substance’s identity (common name, systematic name, formula, molar mass, CAS number), its photographic functions, the processes it appears in, its physical form, its hazard classification, the protective equipment and storage it needs, what it must not be mixed with, and which waste stream it belongs to. Each page also explains, in words, what the substance does in a developer, a fixer, a toner or an emulsion, and links to every formula in the Formulary that contains it.

The chemicals

Section titled “The chemicals”

acid

  • Acetic acid (glacial)BFormula CH3COOHCAS 64-19-7Signal word DangerThe stop bath's acid and the acid fixer's reserve — weak enough to swallow the alkali a print carries out of the developer without throwing sulfur out of the hypo, and the reason development stops the instant the print hits the second tray.
  • Boric acidBFormula H3BO3CAS 10043-35-3Signal word DangerThe very weak acid that holds a hardening fixer inside its narrow pH window — enough acid reserve to swallow the developer carried in, too weak to throw sulfur out of the hypo.
  • Citric acid (anhydrous)AFormula C6H8O7CAS 77-92-9Signal word WarningThe low-odour stop-bath acid that is also a ligand — a triprotic acid whose citrate ion binds iron(III), which is why the same white powder stops a print developing, sensitises a cyanotype and clears a platinum print.
  • Hydrochloric acidBFormula HClCAS 7647-01-0Signal word DangerA strong mineral acid that photographers meet heavily diluted — the clearing bath that strips residual iron from a platinum print, the acid in a chromium intensifier, and half of the aqua regia that makes gold chloride.
  • Nitric acidCFormula HNO3CAS 7697-37-2Signal word DangerThe acid that dissolves silver and so stands behind every silver salt in photography — an oxidising, acutely toxic corrosive whose reaction with metals evolves nitrogen dioxide, and which this course studies rather than handles.
  • Oxalic acidBFormula C2H2O4CAS 144-62-7Signal word DangerThe strongest of the plant acids and the one that makes a platinum print — excess oxalate to keep ferric oxalate in solution, a trace in the developer to hold the pH down, and a systemic poison that has to be kept out of the silver waste.
  • Sulfamic acidAFormula H3NO3SCAS 5329-14-6Signal word WarningA strong acid you can weigh — an odourless, non-volatile crystalline solid that clears an iron print, strips the chalk out of buffered paper, and lends its anion to the one silver salt in alternative printing that is not the nitrate.
  • Sulfuric acidBFormula H2SO4CAS 7664-93-9Signal word DangerThe acid that turns potassium permanganate into a working bleach — a few millilitres per litre in a reversal or reducing bath, and the one reagent in the darkroom whose dilution can boil if it is poured the wrong way round.
  • Tartaric acidBFormula C4H6O6CAS 87-69-4Signal word DangerThe grape acid that makes a Van Dyke brown print work — a chelating dicarboxylic acid whose iron(III) salt is light-sensitive, and which turns up again as the stabiliser in gold toning baths.

alkali

  • Ammonia solution (ammonium hydroxide)BFormula NH4OHCAS 1336-21-6Signal word DangerThe volatile alkali that is also a ligand — it dissolves silver halide, redissolves silver oxide into the diammine complex, ripens an emulsion, raises the sensitivity of a dried plate, and forms a contact explosive if it is left standing with silver.
  • Borax (sodium tetraborate decahydrate)BFormula Na2B4O7·10H2OCAS 1303-96-4Signal word DangerThe mild alkali of D-76 and of the alkaline gold toners — 2 g per litre where a paper developer takes tens of grams of carbonate, because a fine-grain developer must not be hurried.
  • Potassium carbonate (anhydrous)AFormula K2CO3CAS 584-08-7Signal word WarningThe substitute alkali — the same carbonate buffering as the sodium salt but nearly four times as soluble, so it makes the concentrated liquid developer the sodium salt cannot, at the price of being deliquescent enough to weigh badly.
  • Potassium hydroxideBFormula KOHCAS 1310-58-3Signal word DangerThe other strong alkali — chemically interchangeable with caustic soda at 56 parts to 40, still the alkali of the fast hydroquinone line developers, and even more determined to absorb the room.
  • Sodium bicarbonateAFormula NaHCO3CAS 144-55-8Signal word NoneThe mild alkali of the carbonate family — too weak to drive a developer, right for a gold toning bath that must be made faintly alkaline, and the reagent Kodak names for neutralising an acid stop bath before it goes to the drain.
  • Sodium carbonate (anhydrous)AFormula Na2CO3CAS 497-19-8Signal word WarningThe alkali of the classic developer — a reservoir that releases hydroxide as development consumes it, the accelerator whose quantity sets the energy of the bath, and a salt sold in three grades whose strengths differ by a factor of nearly three.
  • Sodium carbonate decahydrateAFormula Na2CO3·10H2OCAS 6132-02-1Signal word WarningWashing soda — the "crystals" of every nineteenth-century formula, ten waters per carbonate so that only 37 per cent of what you weigh is alkali, and a salt that changes its own composition on the shelf.
  • Sodium carbonate monohydrateAFormula Na2CO3·H2OCAS 5968-11-6Signal word WarningThe middle grade of the carbonate alkali — the same buffering ion as the anhydrous salt, carrying one water of crystallisation, so that about 85% of what you weigh is carbonate and a formula mixed with the wrong grade is out by a sixth.
  • Sodium hydroxideBFormula NaOHCAS 1310-73-2Signal word DangerThe strong alkali that drives hydroquinone in a high-contrast developer and sets the pH of a thiourea toner — free hydroxide with no reservoir behind it, and a solid that heats the water it dissolves in.
  • Sodium metaborate (anhydrous)BFormula NaBO2CAS 7775-19-1Signal word WarningThe alkali that sits between borax and sodium carbonate — borax plus caustic soda, in effect — and the anhydrous salt is only two thirds the weight of the hydrate a formula usually means.
  • Sodium metaborate tetrahydrateBFormula NaBO2·4H2OCAS 10555-76-7Signal word DangerThe hydrate actually sold, and the one a formula means by Kodalk — an alkali between borax and carbonate that raises no carbon dioxide, so a film can go from developer to acid fixer in the tropics without blistering.

antifoggant

  • 6-NitrobenzimidazoleCFormula C7H5N3O2CAS 94-52-0Signal word WarningThe first organic antifoggant of the gelatin era — a pale yellow powder whose silver salt is less soluble than silver bromide, measured at the National Bureau of Standards in 1932 and superseded in the darkroom by benzotriazole.
  • BenzotriazoleBFormula C6H5N3CAS 95-14-7Signal word WarningThe organic antifoggant of the modern darkroom — a weakly acidic triazole whose silver salt is so insoluble that analysts use it to weigh silver, and developers use it to keep unexposed grains from developing.
  • TetraazaindeneBFormula C6H6N4OCAS 2503-56-2Signal word WarningThe emulsion stabiliser known as TAI — one molecule with three names, no published dose for a hand-made emulsion at any tier, and the substance Part V replaced with a bromide restrainer for that reason.

binder

  • AlbumenACAS 9006-50-2Signal word NoneEgg white, beaten to a froth, salted, acidified and deliberately allowed to age — the binder that made the glossiest and most widely printed photograph of the nineteenth century, and the one whose own sulfur is now yellowing it.
  • ArrowrootACAS 9005-25-8Signal word NoneThe starch of a West Indian rhizome, boiled to a cream and brushed onto paper — a salted-paper binder that takes no part in the silver chemistry at all, which is exactly why the formula built on it has to carry citric acid.
  • GelatinACAS 9000-70-8Signal word NoneThe collagen-derived protein that holds the silver halide crystals in place, swells in water and sets to a jelly — the only entry in this encyclopaedia with no molecular formula, because gelatin is a specification and not a compound.
  • Gum arabicACAS 9000-01-5Signal word NoneThe acacia exudate that carries pigment in gum bichromate, coats a Pellet cyanotype and sizes a printing paper — a mixture with no formula, a food additive with no GHS classification, and a Level A material inside a process the course studies and does not perform.
  • NitrocelluloseDCAS 9004-70-0Signal word DangerPyroxylin, the nitrated cotton that dissolves in ether and alcohol to make collodion and later became the film base that burned down archives — a study entry, with the chemistry and the fire hazard in full and no procedure at all.

bleach

  • Potassium ferricyanideBFormula K3[Fe(CN)6]CAS 13746-66-2Signal word WarningThe red prussiate of potash — the oxidising bleach that turns image silver into silver ferrocyanide, and so drives Farmer's reducer, the sepia bleach, the metal-ferrocyanide toners and the traditional cyanotype.

buffer

  • Calcium carbonateAFormula CaCO3CAS 471-34-1Signal word NoneChalk — the buffer that archival paper standards put into every sheet, the reason a cyanotype fades on a good mount, and one of the very few substances in this encyclopaedia that the course spends more effort removing than using.
  • Sodium acetate (anhydrous)AFormula CH3COONaCAS 127-09-3Signal word NoneThe alkali of the oldest gold toning bath — a salt that has to stand for twenty-four hours before it will work, the textbook partner of acetic acid in a buffer, and a name a conservator may have misread for a century.

developing agent

  • AmidolBFormula C6H10Cl2N2OCAS 137-09-7Signal word DangerThe one developing agent that works without an alkali — 2,4-diaminophenol dihydrochloride, which develops in nothing but sulfite, keeps for about a week once wet, and is classified toxic if swallowed.
  • Ascorbic acidAFormula C6H8O6CAS 50-81-7Signal word NoneVitamin C as a developing agent — the reducing sugar-lactone behind Kodak's XTOL and the hydroquinone-free developers, whose driving force rises with pH, whose solutions oxidise in a partly filled bottle, and which no notifier to the European inventory classifies as hazardous.
  • CatecholCFormula C6H6O2CAS 120-80-9Signal word DangerHydroquinone's ortho isomer and the staining, tanning developing agent of pyrocatechin formulas — benzene-1,2-diol, notified as Danger with an acute-toxicity pictogram, a skin notation and a carcinogenicity statement, and handled by this course at Level C.
  • ChlorohydroquinoneBFormula C6H5ClO2CAS 615-67-8Signal word DangerAdurol — hydroquinone with one chlorine on the ring, which develops faster, minds the cold less and keeps better, and which the aggregated notifications classify toxic in contact with skin.
  • Dimezone SAFormula C11H14N2O2CAS 13047-13-7Signal word WarningThe pyrazolidone in every current ILFORD paper developer — phenidone with two groups added to one carbon, named on the packet and explained nowhere, and the substance a home worker never weighs.
  • Gallic acidBFormula C7H6O5CAS 149-91-7Signal word DangerThe developing agent of the calotype — 3,4,5-trihydroxybenzoic acid, barely soluble in cold water, one carboxyl group away from pyrogallol, and the substance that made Talbot's latent image visible.
  • GlycinBFormula C8H9NO3CAS 122-87-2Signal word WarningThe clean, slow, neutral-toned developing agent of stand and tank development — N-(4-hydroxyphenyl)glycine, sold by Kodak Limited as Kodurol, whose development time is very nearly proportional to its dilution and which oxidises to a useless brown powder in an open tin.
  • HydroquinoneBFormula C6H6O2CAS 123-31-9Signal word DangerThe contrast-building developing agent of every MQ and PQ developer — benzene-1,4-diol, the slowest starter and the steadiest density-maker, classified as a suspected carcinogen and mutagen with a workplace exposure limit of 0.5 mg per cubic metre.
  • Iron(II) oxalateBFormula FeC2O4CAS 516-03-0Signal word WarningThe yellow, insoluble salt that develops a plate by giving up one electron — the active agent of the ferrous-oxalate developer, which is almost never bought, and the photoproduct that makes every iron printing process need a developer at all.
  • Iron(II) sulfate (anhydrous)AFormula FeSO4CAS 7720-78-7Signal word WarningThe waterless reference form of the iron developer salt — the same reducing iron(II) that develops a collodion plate and makes ferrous oxalate, but without the seven waters every photographic formula assumes, so weighing it against an old recipe puts nearly twice the iron in the tray.
  • Iron(II) sulfate heptahydrateAFormula FeSO4·7H2OCAS 7782-63-0Signal word WarningGreen vitriol, the blue-green crystal that develops a wet collodion plate by plating silver out of the solution already on the film, and the salt from which the ferrous oxalate developer is made.
  • MercuryDFormula HgCAS 7439-97-6Signal word DangerThe liquid metal whose vapour brought out an invisible exposure on a daguerreotype plate by amalgamating with the silver where light had acted — a study entry, with the chemistry and the hazard given in full and no procedure given at all.
  • MetolBFormula (C7H9NO)2·H2SO4CAS 55-55-0Signal word WarningThe fast, detail-giving developing agent of the classic metol–hydroquinone developer — 4-(methylamino)phenol hemisulfate, sold as Elon and Genol, and the substance that gave darkroom dermatitis its name.
  • p-AminophenolBFormula C6H7NOCAS 123-30-8Signal word WarningThe developing agent behind Rodinal and every liquid concentrate like it — 4-aminophenol, barely soluble as the free base, freely soluble as a salt or in strong alkali, and the parent of the whole metol family.
  • PhenidoneBFormula C9H10N2OCAS 92-43-3Signal word WarningIlford's wartime replacement for metol — 1-phenylpyrazolidin-3-one, a developing agent that does metol's job at a fifth to a sixth of the weight and supplies the P of every PQ developer.
  • PyrogallolBFormula C6H6O3CAS 87-66-1Signal word WarningThe oldest developing agent still in production, and the only one whose oxidation product is deliberately left in the negative — benzene-1,2,3-triol, the pyro of pyro-ammonia and of PMK, whose yellow-green stain is a second image and whose tanning action hardens the gelatin it develops.
  • Sodium ascorbateAFormula C6H7NaO6CAS 134-03-2Signal word NoneThe sodium salt of vitamin C — the form that puts the ascorbate anion into a developer without taking the alkali away with it, nearly twice as soluble as the free acid, and oxidising faster in the bottle because of the very pH that makes it convenient.

fixing agent

  • Ammonium thiosulfateAFormula (NH4)2S2O3CAS 7783-18-8Signal word NoneThe fixing agent in every bottle labelled "rapid fixer" — the same thiosulfate ion as hypo with ammonium in place of sodium, sold as a concentrated solution that fixes film in two to five minutes and gives off ammonia if it is ever made alkaline.
  • Potassium cyanideDFormula KCNCAS 151-50-8Signal word DangerThe wet-plate photographer's other fixer, faster than hypo and leaving a whiter collodion positive, whose solvent power came with acute toxicity by four routes — a substance the course studies and never handles.
  • Sodium cyanideDFormula NaCNCAS 143-33-9Signal word DangerThe sodium half of the cyanide pair — the same dissolving power over silver halide as the potassium salt, the same gas from any acid, a tighter British exposure limit, and a substance the course studies and never handles.
  • Sodium thiosulfate (anhydrous)AFormula Na2S2O3CAS 7772-98-7Signal word NoneThe fixing agent in its water-free grade — the ion that dissolves silver halide by wrapping the silver in a soluble complex, carrying about 1.57 times as much thiosulfate per gram as the hypo crystals most published formulas were written for.
  • Sodium thiosulfate pentahydrateAFormula Na2S2O3·5H2OCAS 10102-17-7Signal word NoneHypo — the big watery crystals nearly every historical fixer, sepia toner and reducer was weighed in, a third of whose mass is water of crystallisation and which chills the beaker as it dissolves.

halide source

  • Ammonium bromideBFormula NH4BrCAS 12124-97-9Signal word DangerThe bromide that brings its own ammonium ion — the same halide as potassium bromide at four-fifths of the weight, a gentler emulsion, and the only ordinary darkroom salt in this family carrying a harmonised reproductive-toxicity classification.
  • Ammonium chlorideAFormula NH4ClCAS 12125-02-9Signal word WarningSal ammoniac — the chloride nineteenth-century printers beat into their albumen, an acidic salt that both salts the paper and helps denature the egg white, and the one halide in this family with a workplace exposure limit.
  • Cadmium bromideDFormula CdBr2CAS 7789-42-6Signal word WarningThe bromide the collodion era chose for its solubility in alcohol and the stability it gave a salted collodion — the bromide in Maddox's first gelatin emulsion, and a cadmium compound the course studies and never handles.
  • Lithium chlorideBFormula LiClCAS 7447-41-8Signal word WarningThe chloride that dissolves in alcohol and ether, which is why the collodion printing-out papers were salted with it — and which today earns its place by holding a sealed box at 15 per cent relative humidity.
  • Potassium chlorideAFormula KClCAS 7447-40-7Signal word NoneThe chloride of the gaslight paper emulsion — three grams of it against five of silver nitrate, an ECHA entry that says the substance meets no hazard criterion at all, and a molar mass that makes it not quite interchangeable with table salt.
  • Sodium chlorideAFormula NaClCAS 7647-14-5Signal word WarningCommon salt — the chloride that makes salted paper, the cheap halide of a chloride emulsion, and the reagent that turns a silver nitrate solution into light-sensitive silver chloride inside the paper's own fibres.

hardener

  • Chrome alumBFormula KCr(SO4)2·12H2OCAS 7788-99-0Signal word WarningChromium(III) potassium sulfate dodecahydrate — the violet salt that hardens gelatin three times as efficiently as potash alum, subbed the first dry plates to their glass, and loses its hardening power in days once sulfite reaches it.
  • FormaldehydeCFormula CH2OCAS 50-00-0Signal word DangerThe simplest aldehyde, sold to photographers as formalin — the most powerful gelatin hardener the formularies carry, notified as Danger with four pictograms and a carcinogenicity statement, and handled by this course at Level C.
  • GlutaraldehydeCFormula C5H8O2CAS 111-30-8Signal word DangerPentanedial — the two-ended aldehyde proposed as a lower-volatility replacement for formalin in hardening gelatin, and a respiratory sensitiser with a British workplace exposure limit of 0.05 ppm, handled by this course at Level C.
  • GlyoxalCFormula C2H2O2CAS 107-22-2Signal word WarningThe smallest dialdehyde, sold as a 40 per cent solution and used to harden gelatin — the low-volatility replacement for formalin in paper sizing, and the hardener Part V rejected for having no dose and a harmonised mutagenicity statement.
  • Potassium alumAFormula KAl(SO4)2·12H2OCAS 7784-24-9Signal word NoneThe dodecahydrate double sulfate of potassium and aluminium — the tanning agent of the acid hardening fixer, the acid half of a hypo-alum sepia toner, and the size hardener that made British calotype paper behave differently from French.
  • Tannic acidAFormula C76H52O46CAS 1401-55-4Signal word WarningThe preservative of Major Russell's dry collodion plate and a hardener of gelatin — not one substance but a family of gallotannins, whose published formula has moved from 618 to 1701 without the jar changing.

intensifier

  • Lead(II) nitrateDFormula Pb(NO3)2CAS 10099-74-8Signal word DangerThe soluble lead salt that, with potassium ferricyanide, gave the strongest intensifier of the 1870s and a family of coloured toners — and, through Eder's study of it, the chemical basis of ferricyanide toning itself.
  • Mercury(II) chlorideDFormula HgCl2CAS 7487-94-7Signal word DangerCorrosive sublimate, the white salt that bleached a silver negative to a mixture of silver and mercurous chlorides so it could be redeveloped denser — photography's standard intensifier for eighty years, and a substance the course studies and never handles.

light-sensitive salt

  • Silver bromideBFormula AgBrCAS 7785-23-1Signal word DangerThe halide of the negative — a pale yellow, almost insoluble salt that carries the latent image in every film and enlarging paper, is made in the dark from silver nitrate and a soluble bromide, and is dissolved away again by thiosulfate at the end.
  • Silver chlorideBFormula AgClCAS 7783-90-6Signal word WarningThe first photographic substance and the slowest of the three halides — a white, nearly insoluble salt formed in place from silver nitrate and a chloride, which prints out a warm brown image in ultraviolet and violet light and makes the contact papers that need no enlarger.
  • Silver iodideBFormula AgICAS 7783-96-2Signal word WarningThe most insoluble and the most stubborn of the three photographic halides — the sensitive layer of the daguerreotype, the calotype and the wet collodion plate, and, in trace amounts, the additive that makes a modern bromide emulsion clean and fast.

metal salt

  • Iron(III) chloride (anhydrous)BFormula FeCl3CAS 7705-08-0Signal word DangerThe deliquescent black-brown salt of the photogravure etching bath and of sensitisers built from scratch — an iron(III) source whose solution is not a neutral salt but a strong acid, and which must never be confused by weight with the yellow hexahydrate a laboratory actually stocks.
  • Iron(III) chloride hexahydrateBFormula FeCl3·6H2OCAS 10025-77-1Signal word DangerThe yellow-brown salt a formula means when it says "ferric chloride" — Herschel's blue-developing bath of 1842, the gum hardener of the positive-working blueprint, an ingredient of the fastest commercial blueprint papers, and the iron reducer that Wall told photographers to stop using.
  • Iron(III) nitrate nonahydrateBFormula Fe(NO3)3·9H2OCAS 7782-61-8Signal word DangerThe plain, well-defined iron salt that Mike Ware's Simple Cyanotype builds its sensitiser out of — a deliquescent oxidising solid, two fifths water by weight, which must never be dissolved in water alone, and the salt with which John Mercer printed light onto cloth in 1828.
  • Silver sulfamateBFormula NH2SO3AgSignal word NoneThe one silver salt in alternative printing that is not the nitrate — a compound with no CAS number, no supplier and no classification, made in a beaker out of silver oxide and a descaling acid so that an iron-silver sensitiser can work on the acid side.
  • Silver(I) oxalateDFormula Ag2C2O4CAS 533-51-7Signal word DangerThe precipitate a kallitype sensitiser is expected to throw — a white silver salt nobody sells, that most of its notifiers classify as an explosive, and that the course teaches so a reader knows never to let a mixed oxalate-and-silver bottle dry out.
  • Silver(I) oxideBFormula Ag2OCAS 20667-12-3Signal word DangerThe way to put silver into a bottle without putting an anion in with it — a brownish-black powder that is 93 per cent silver by weight, the base nineteenth-century photographers made in the beaker rather than bought, and the ingredient that lets Mike Ware's argyrotype work on the acid side.

other

  • Barium sulfateAFormula BaSO4CAS 7727-43-7Signal word NoneBaryta — the white mineral layer between the paper and the emulsion of every fibre-base print, and the one barium compound that is not treated as a poison, because almost nothing dissolves it.
  • Bitumen of JudeaDSignal word NoneThe natural asphalt that hardens where light falls, the image substance of the earliest surviving camera photograph, and the one entry in this encyclopaedia with no formula, no molar mass and no CAS number the course is willing to assign it.
  • GlycerolAFormula C3H8O3CAS 56-81-5Signal word NoneA syrup that does no photographic chemistry at all — it holds water in a drying coating and thickens a developer to a crawl, and those two physical properties decide the colour of an argyrotype and made local development of a platinum print possible.
  • KaolinACAS 1332-58-7Signal word NoneChina clay, and the only maintenance a silver sensitising bath has — about 15 g per litre left in the storage bottle, shaken up after every session, adsorbing the dissolved organic matter that turns the solution brown and then black.
  • Prussian blueAFormula Fe4[Fe(CN)6]3CAS 14038-43-8Signal word WarningThe image substance of every cyanotype — a mixed-valence iron cyanide whose colour comes from electrons hopping between two oxidation states, whose composition is a family rather than a formula, and whose one weakness is alkali.
  • SilverAFormula AgCAS 7440-22-4Signal word WarningThe metal the whole course is named for — noble enough to survive a century in a shoebox, tarnished by a trace of sulfur in the air, and coloured by nothing but the size of the particles it was reduced into.
  • Silver(I) selenideAFormula Ag2SeCAS 1302-09-6Signal word NoneThe compound a selenium toner leaves in the print — the reason the treatment is called archival, the reason the colour shift can be almost nothing, and a substance about which the course holds far less data than it would like.
  • Silver(I) sulfideAFormula Ag2SCAS 21548-73-2Signal word NoneWhat sepia toning makes, what tarnish is, and what yellows the highlights of an under-fixed albumen print — one compound, forty orders of magnitude less soluble than silver chloride, doing all three jobs.
  • Sodium sulfate (anhydrous)AFormula Na2SO4CAS 7757-82-6Signal word NoneWhat sodium sulfite turns into when it oxidises — one letter and one digit away from the preservative it replaces, useless in its place, and yet a real ingredient of a tropical developer and a blue toner.
  • Titanium dioxideAFormula TiO2CAS 13463-67-7Signal word WarningThe white pigment in the polyethylene layer of every resin-coated print, and the additive a siderotype printer must not have in the paper — insoluble, harmonised as a suspected carcinogen only as a fine powder, and classified three different ways by three regulators.

oxidiser

  • Ammonium persulfateCFormula (NH4)2S2O8CAS 7727-54-0Signal word DangerThe only flattening reducer Kodak knew of, and the salt R-1 and R-5 name by name — eleven times more soluble than the potassium persulfate it is constantly confused with, and a respiratory sensitiser with a 0.1 mg per cubic metre limit.
  • Calcium nitrate tetrahydrateBFormula Ca(NO3)2·4H2OCAS 13477-34-4Signal word WarningThe salt that holds a lidded box at 55 per cent relative humidity — an instrument rather than a reagent, which never touches the paper, decides the colour of a print-out palladium image, and is an oxidiser that must be kept off anything that burns.
  • Hydrogen peroxideAFormula H2O2CAS 7722-84-1Signal word DangerThe one darkroom reagent that comes from a chemist's shop — an oxidising bath that turns a washed cyanotype blue in thirty seconds, and whose published classification describes a concentrate nobody in this course will ever open.
  • Potassium chlorateDFormula KClO3CAS 3811-04-9Signal word DangerThe strong oxidiser that platinotype printers dropped into a ferric oxalate sensitiser to raise contrast, by destroying some of the iron(II) that light had just made — a study entry, and a contrast control the course replaces with a calibrated negative.
  • Potassium permanganateCFormula KMnO4CAS 7722-64-7Signal word DangerThe dark purple crystal that oxidises silver to soluble silver sulphate in acid and does almost nothing without it — Kodak's R-2 and R-5, ILFORD's reversal bleach, the hypo test, and a strong oxidiser whose exposure limits are quoted as manganese and disagree fivefold.
  • Potassium persulfateCFormula K2S2O8CAS 7727-21-1Signal word DangerAnthion, the hypo eliminator, and the persulfate in Kodak's Nelson gold toner — a strong oxidiser sold for a century as a darkroom convenience, and a respiratory sensitiser with a workplace limit of 0.1 mg per cubic metre.

preservative

  • Potassium metabisulfiteAFormula K2S2O5CAS 16731-55-8Signal word DangerThe potassium twin of sodium metabisulfite — the salt Mawson and Swan introduced in 1886 to keep pyrogallol developers, still specified in acid clearing baths, and the one that costs the most and delivers the least bisulfite per gram.
  • Sodium bisulfiteAFormula NaHSO3CAS 7631-90-5Signal word WarningThe acid member of the sulfite family — a preservative for stock developer solutions that keep better acid than alkaline, and the salt that supplies a fixing bath with both its sulfite and its acid at once.
  • Sodium metabisulfiteAFormula Na2S2O5CAS 7681-57-4Signal word DangerThe dry, keepable form of bisulfite — a sulfite-and-acid preservative in one powder that becomes sodium bisulfite the moment it dissolves, and the salt actually inside most tubs labelled sodium bisulfite.
  • Sodium sulfite (anhydrous)AFormula Na2SO3CAS 7757-83-7Signal word DangerThe preservative in almost every black-and-white developer and fixer — an oxygen scavenger that keeps the solution from browning, a trap that turns oxidised developer into colourless sulfonates, and, at high concentration, a mild silver halide solvent that gives fine grain.
  • ThymolBFormula C10H14OCAS 89-83-8Signal word DangerThe antiseptic that keeps gelatin from putrefying — a crystalline phenol from thyme, dosed from alcohol because it is barely soluble in water, and classified corrosive and very toxic to aquatic life.

reducing agent

  • Ammonium sulfiteAFormula (NH4)2SO3CAS 10196-04-0Signal word DangerThe sulfite in the German selenium toner — a flue-gas commodity that photography borrows, a reducing agent that can give up either sulfur dioxide or ammonia depending on which way you push it, and a name that three databases attach to three different substances.

restrainer

  • Potassium bromideAFormula KBrCAS 7758-02-3Signal word WarningThe restrainer of the black-and-white developer — a soluble bromide that holds back fog, warms the tone of a print, supplies the halide when an emulsion is precipitated, and turns the silver of a finished print back into silver bromide in the sepia bleach.
  • Potassium iodideBFormula KICAS 7681-11-0Signal word DangerThe minority halide — a fraction of a gram that makes a bromo-iodide emulsion cleaner, joins bromide as a restrainer in the developer, and dissolves the iodine that water will not.

sensitiser

  • Ammonium dichromateDFormula (NH4)2Cr2O7CAS 7789-09-5Signal word DangerThe more soluble of the two photographic dichromates, preferred for gum printing and photomechanical work and still sold as a contrast drop for iron processes — a chromium(VI) compound the course studies and never handles.
  • Ammonium iron(III) citrateACAS 1185-57-5Signal word WarningThe light-sensitive iron salt of the classic cyanotype, the Van Dyke and the argyrotype — and not a compound at all, but a variable mixture of iron, citrate and ammonium whose iron content ranges from 14 to 28 per cent, so that no two printers working apart can be certain they hold the same substance.
  • Ammonium iron(III) oxalateBFormula (NH4)3[Fe(C2O4)3]CAS 14221-47-7Signal word WarningThe well-behaved light-sensitive iron salt — bright green crystals of a known structure whose photoproduct is soluble, which is why it prints out instead of needing a developer and why it replaced ferric oxalate in the modern siderotypes and ferric ammonium citrate in New Cyanotype.
  • BromineDFormula Br2CAS 7726-95-6Signal word DangerThe dark red fuming liquid whose vapour, added to iodine, made the daguerreotype fast enough for a portrait — the parent of every bromide in this course, and a substance the course studies and never handles as the element.
  • ChlorineDFormula Cl2CAS 7782-50-5Signal word DangerThe greenish-yellow gas that accelerated the daguerreotype, that Berthollet found light-sensitive in water in 1785, and that a darkroom can make by accident from bleach and acid — a study entry, with the chemistry and the hazard and no procedure.
  • ErythrosinBFormula C20H8I4O5CAS 15905-32-5Signal word NoneThe orthochromatic sensitising dye — tetraiodofluorescein, the food colour E127, unclassified under GHS on a thin evidence base, and the substance whose FDA authorisation for food was revoked in January 2025.
  • Ferric oxalateBFormula Fe2(C2O4)3CAS 2944-66-3Signal word WarningThe light-sensitive iron(III) salt that makes a platinum, palladium or kallitype print — polymeric, polymorphic, of uncertain water content and famously variable from supplier to supplier, which is why every one of its formulas is really a formula for a solution rather than for a solid.
  • IodineDFormula I2CAS 7553-56-2Signal word WarningThe violet element that sublimes at room temperature and converts the surface of a silver plate into silver iodide — the daguerreotype's sensitiser, and a substance the course studies and never handles as the element.
  • Palladium(II) chlorideBFormula PdCl2CAS 7647-10-1Signal word DangerThe cheap precursor of every palladium printing solution — a dark brown powder that will not properly dissolve in water until chloride is added to complete the tetrachloropalladate ion, and whose worst hazard is its dust.
  • Potassium dichromateDFormula K2Cr2O7CAS 7778-50-9Signal word DangerThe orange-red salt that made gum, carbon and photomechanical printing possible by hardening a colloid where light struck it, and a chromium(VI) compound the course studies and never handles.
  • Potassium tetrachloropalladate(II)BFormula K2[PdCl4]CAS 10025-98-6Signal word DangerThe potassium salt of the tetrachloropalladate ion — the same image-forming anion that makes a palladium print, bought from a chemical catalogue rather than a photographic one, and classified as a skin sensitiser despite palladium's reputation beside platinum.
  • Potassium tetrachloroplatinate(II)CFormula K2[PtCl4]CAS 10025-99-7Signal word DangerThe platinum salt of the platinotype — potassium chloroplatinite, the ruby-red compound Willis made for himself in 1873, and a respiratory sensitiser whose first recorded victims were the workers who coated the paper.
  • Silver nitrateBFormula AgNO3CAS 7761-88-8Signal word DangerThe only soluble silver salt photography uses — the source of every silver halide crystal, the sensitiser of salted and albumen paper, and the one reagent in a home darkroom that is corrosive, oxidising, light-reduced and very toxic to aquatic life at once.
  • Sodium tetrachloropalladate(II)BFormula Na2[PdCl4]CAS 13820-53-6Signal word DangerThe palladium salt the photographic trade actually sells — "palladium solution No. 3", sodium chloropalladite, sold as a 15 per cent solution and made either from the solid salt or by dissolving palladium chloride in brine.

sequestering agent

  • Ammonium citrate (dibasic)AFormula (NH4)2HC6H5O7CAS 3012-65-5Signal word WarningThe mildly acidic citrate that develops and clears a palladium print without touching the paper — and the one salt in this family whose label has to be read twice, because two different ammonium citrates are sold under the same name.
  • EDTA disodium salt (anhydrous)AFormula C10H14N2Na2O8CAS 139-33-3Signal word NoneThe other article sold as disodium EDTA — same ligand, same job, 10.7 per cent lighter per gram than the dihydrate, and the reason a clearing bath measured by molarity and one measured by the tablespoon are two different kinds of instruction.
  • EDTA disodium salt dihydrateAFormula C10H14N2Na2O8·2H2OCAS 6381-92-6Signal word NoneThe hexadentate ligand that lifts iron out of an iron-based print and holds calcium out of the way in hard water — a mildly acidic sodium salt of ethylenediaminetetraacetic acid, and the modern replacement for hydrochloric acid clearing.
  • Potassium oxalate (anhydrous)BFormula K2C2O4CAS 583-52-8Signal word WarningThe waterless form of the platinum developer's salt — chemically the same oxalate ion that dissolves ferrous oxalate and iron blues, but 166.22 to the mole against 184.23, so a recipe weighed out with the wrong one carries a tenth more oxalate than it should.
  • Potassium oxalate monohydrateBFormula K2C2O4·H2OCAS 6487-48-5Signal word WarningThe platinum printer's developer, which contains no developing agent at all — a salt whose oxalate ions dissolve the insoluble ferrous oxalate that light has made, so that the iron(II) can move and reduce the platinum; also the solvent that keeps iron out of a sepia bleach.
  • Potassium sodium tartrate tetrahydrateAFormula KNaC4H4O6·4H2OCAS 6381-59-5Signal word NoneRochelle salt — the tartrate developer that decides whether a kallitype comes out black or sepia, a salt that gets cold as it dissolves, and one of the few reagents in this encyclopaedia that three separate ECHA entries agree meets no hazard criterion at all.
  • Sodium hexametaphosphateAFormula (NaPO3)6CAS 10124-56-8Signal word NoneThe glassy polyphosphate that keeps calcium out of the way — sold as Calgon, listed by Kodak among its tested chemicals, and still specified by Foma to make a reversal bath work in hard water.
  • Triammonium citrateAFormula (NH4)3C6H5O7CAS 3458-72-8Signal word WarningThe fully neutralised ammonium salt of citric acid, weighed out of a jar to make a cyanotype sensitiser that would otherwise have to be titrated with ammonia — and the one substance in the encyclopaedia most likely to be bought by mistake, because a different salt is sold under the same two words.
  • Tripotassium citrate monohydrateAFormula K3C6H5O7·H2OCAS 6100-05-6Signal word NoneThe tribasic salt that holds iron(III) in solution so it can reduce a negative without staining it, restrained a developer for the Victorians, and carries no GHS classification at all — on an evidence base that deserves reading before it is trusted.
  • Trisodium citrate dihydrateAFormula Na3C6H5O7·2H2OCAS 6132-04-3Signal word NoneThe citrate that develops a palladium print, develops a kallitype, and goes into a salting solution to absorb the chlorine a printing-out paper releases — one salt doing three unrelated jobs, all of them by holding on to something.

silver halide solvent

  • Potassium thiocyanateBFormula KSCNCAS 333-20-0Signal word DangerA silver solvent mild enough to put in a developer at one gram a litre — the fine-grain agent of Kodak's DK-20, a gold-toner ligand, and a salt that gives off cyanide if you ever let it meet an acid or a flame.

solvent

  • Diethyl etherDFormula (C2H5)2OCAS 60-29-7Signal word DangerThe volatile half of the collodion solvent pair — extremely flammable, heavier than air, an anaesthetic in its own right and a former of explosive peroxides on the shelf, and a substance the course studies and never handles.
  • EthanolBFormula C2H6OCAS 64-17-5Signal word DangerThe solvent of the collodion era and the drying aid of the gelatine one — the alcohol that dissolves pyroxylin, washes an emulsion by taking its water, draws a coating to the edge of a plate and dries a negative in five minutes.
  • Isopropyl alcoholAFormula C3H8OCAS 67-63-0Signal word DangerThe one volatile organic solvent this course's safety rubric admits to a home darkroom — a water-miscible alcohol used at 70 per cent to coagulate an albumen layer, to clean glass, and to pull water out of a gelatin film.
  • Oil of lavenderDCAS 8000-28-0Signal word NoneThe essential oil that dissolved unhardened bitumen off Niépce's plates, and whose distillation residue turns out to be a light-sensitive resin in its own right — a study entry, and the substance in this encyclopaedia with the thinnest hazard record of all.

toner

  • Ammonium thiocyanateBFormula NH4SCNCAS 1762-95-4Signal word DangerThe thiocyanate the alternative-process gold toners are sold with — a 2 per cent stock beside a 0.2 per cent gold, a silver solvent, and the salt whose datasheet says it contains no cyanide and whose old classification says an acid will make some.
  • Chloroauric acidBFormula HAuCl4CAS 16903-35-8Signal word DangerThe substance sold to photographers as "gold chloride" — a deliquescent, corrosive acid of gold(III) that is the starting material for every gold toner, for Herschel's chrysotype and for Ware's modern gold printing process.
  • Copper(II) sulfateBFormula CuSO4CAS 7758-98-7Signal word WarningThe blue crystal of the copper toners and the cupric bleaches — a salt whose two grades differ by more than half again in weight, whose classification changes signal word between them, and which one historical process pairs with a chromium(VI) compound the course will not use.
  • Gold(III) chlorideBFormula AuCl3CAS 13453-07-1Signal word DangerThe true chloride of gold, AuCl3 — the compound photographers name on every toning formula and almost never actually have, since "gold chloride" of commerce is chloroauric acid, and the difference is a third again as much gold per gram.
  • Lead(II) acetateDFormula Pb(CH3COO)2CAS 301-04-2Signal word DangerSugar of lead — the salt that scavenges sulfide out of a gold toning bath, drops lead oxalate into a platinum sensitiser and turns a cyanotype violet by climbing inside the Prussian blue lattice. It is also the one study-only substance in this course that a beginner can still be sold today.
  • Nickel(II) sulfateDFormula NiSO4CAS 7786-81-4Signal word DangerThe one cyanotype toner whose real purpose is not colour but survival — it makes Prussian blue about ten times harder for alkali to destroy. It is also a Category 1A carcinogen by inhalation and the salt dermatologists patch-test with, and the course does not put it in a darkroom.
  • Potassium ferrocyanide (anhydrous)BFormula K4[Fe(CN)6]CAS 13943-58-3Signal word NoneYellow prussiate of potash — the reduced partner of the red prussiate, the salt every ferricyanide bleach makes for itself, and the reagent that turns a silver image into Prussian blue.
  • Potassium ferrocyanide trihydrateBFormula K4[Fe(CN)6]·3H2OCAS 14459-95-1Signal word NoneThe pale yellow crystals a supplier actually ships — yellow prussiate of potash with three waters of crystallisation, the form nineteenth-century formulas mean, and the one that loses weight in a dry cupboard.
  • SeleniumDFormula SeCAS 7782-49-2Signal word DangerThe element behind every selenium toner — insoluble in water, toxic if swallowed or inhaled, dissolved in the old formulas by boiling it with sodium sulfide or fusing it with caustic soda, and reached today only through a bought, dilute liquid.
  • Sodium selenateCFormula Na2SeO4CAS 13410-01-0Signal word DangerSelenium(VI), the oxidation state the encyclopaedia did not plan for — the salt one European maker names by CAS number in the toner he sells, stable in exactly the alkaline conditions a toning bath provides, and the form of selenium a WHO monograph calls potentially the most environmentally dangerous.
  • Sodium seleniteCFormula Na2SeO3CAS 10102-18-8Signal word DangerThe soluble selenium(IV) salt of the toning family — a dietary supplement at microgram doses and classified fatal if swallowed as a weighed solid, and the entry where this course records what it could not verify about selenium toners.
  • Sodium selenosulfateCFormula Na2SeSO3CAS 25468-09-1Signal word DangerSelenium dissolved in sulfite — the selenium twin of hypo, named in the emulsion literature as a chemical sensitiser at one part in a million, and the compound most often assumed, without proof, to be what is in a bottle of selenium toner.
  • Sodium sulfideCFormula Na2SCAS 1313-82-2Signal word DangerThe redeveloper of the classical sepia toner — the salt that turns a bleached silver image into silver sulfide, and the one darkroom chemical whose reaction with an acid makes a gas your nose stops warning you about.
  • Sodium thiocyanateBFormula NaSCNCAS 540-72-7Signal word DangerThe thiocyanate the gold toners are built on — the ligand in Kodak's GP-1 protective solution and in Reilly's toner for albumen and salted paper, a silver solvent in its own right, and a salt that must never meet an acid.
  • Thallium(I) sulfateDFormula Tl2SO4CAS 7446-18-6Signal word DangerThe cyanotype toner that turns the print cornflower blue by climbing into the same lattice site that makes Prussian blue the antidote for thallium poisoning. Colourless, odourless, tasteless, soluble, and never once handled in this course.
  • ThioureaCFormula CH4N2SCAS 62-56-6Signal word WarningThe odourless sepia toner and the stabiliser in gold blue-toning baths — urea with its oxygen swapped for sulfur, classified as a suspected carcinogen and reproductive toxin, which is why this course teaches it as chemistry and gives no procedure for a home darkroom.
  • Uranyl nitrate hexahydrateDFormula UO2(NO3)2·6H2OCAS 13520-83-7Signal word DangerThe yellow uranium salt that, with potassium ferricyanide, turned a silver image red-brown and intensified it at the same time — photography's one radioactive reagent, studied here and never handled.