Permanence and Image Deterioration

A print does not fail all at once, and it does not fail in one way. A general yellow across the paper, a stain confined to the highlights, and a blue metallic sheen creeping in from the edges are three different diagnoses with three different causes, and two of the three were decided in the darkroom by whoever made the print. The third was decided by whoever framed it. Learning to tell them apart is the closest thing photography has to reading its own medical notes.

Two clocks, running at once

Permanence is a race between what you left in the material and what the environment brings to it.

What you left in is finite and it is yours. Thiosulfate the wash did not reach; silver complexes the fixer dissolved and did not carry away; and, if fixing failed rather than washing, the insoluble first compound Kodak’s 1924 primer called invisible. All three are sulfur and silver sitting next to an image made of silver.

What the environment brings never stops. Oxidant gases, sulfiding gases, acids formed from combustion products and water vapour, peroxides off a varnished frame, and moisture and heat to speed every one of them up. The Library of Congress lists the four families in one sentence — oxidant gases, particulate matter, acidic and sulfiding gases, and environmental fumes — and names ozone and the nitrogen oxides as the two that most threaten a photographic image.

The two clocks interact rather than add. Reilly’s account of when albumen prints begin to yellow puts the same three variables in one sentence: the onset “is probably affected primarily by the moisture level and temperature of the storage environment and the amount of residual thiosulfate and silver-thiosulfate complexes present”. Storage does not decide whether a badly washed print fails; it decides how soon.

Three routes to a damaged image, drawn on one grain

1. 1Sulfiding, from what you left in — residual thiosulfate supplies labile sulfur; image silver becomes silver sulfide; the print yellows and loses density overall
2. 2Highlight staining, from what the fixer left — retained silver in areas that carry no image meets the same sulfur; the whites stain while the image looks unchanged
3. 3Silver mirroring, from what the air brings — an oxidant takes an electron, the mobile ion migrates to the surface and is reduced again there, depositing a bluish metallic sheen strongest at the edges

Drawn to show which route produces which appearance, not to scale and not a micrograph. The mechanism of the third route is the Image Permanence Institute's own: oxidation fades the image, the faded silver migrates to the surface of the print, and other pollutants convert it back into metallic silver.

Sulfiding: what residual thiosulfate does

Sulfiding is the failure the whole of the washing lesson exists to prevent, and its chemistry is short. Thiosulfate is a sulfur carrier. Left in the gelatin it is not stable indefinitely — Part XI showed the acid route in full, in which thiosulfuric acid falls apart and throws sulfur out of solution — and finely divided silver sitting beside a source of labile sulfur becomes silver sulfide.

2 Ag + S → Ag₂S

Sulfiding, as the conservation literature states it in words

Reilly’s sentence is the one that equation is the balanced form of: the presumed mechanism of yellowing is “the formation of silver sulfide by reaction of the … silver with labile sulfur supplied by residual fixer or atmospheric pollution”. Note the two suppliers in that sentence, because the same product arrives by two roads and the print cannot tell you which.

What it looks like is a general yellow-brown, with density lost from the image and the whites no longer white. It is not confined to any part of the tonal scale, because the thiosulfate was not confined either. And it is slow enough to be invisible year to year and obvious decade to decade: Reilly records albumen prints that yellowed within one or two years of processing and others that took very much longer, which is exactly the spread you would expect from a failure whose rate depends on how much was left and on where the print has lived.

📷Why the same fault reads differently on a negative and a print

On a negative the loss is of density, and a thin negative prints as a flat one. On a print the loss is of density and the gain is of stain, so the visible change is larger for the same chemistry: the image goes down and the paper comes up to meet it. That is one reason the historical literature is full of complaints about fading prints and comparatively quiet about fading negatives, and it is why the AIC’s condition description for dry plates separates the two symptoms by how you look at them — mirroring in reflected light, “yellow or brown stains, visible in transmitted light” from improperly washed processing chemistry.

Residual silver: the stain that lives in the highlights

A second failure looks different because its silver is somewhere else.

Residual silver is silver the fixer dissolved but did not carry away, or silver the material held so tightly that the fixer never dissolved it at all. It is distributed with the unexposed halide rather than with the image, which means it is densest exactly where the picture is lightest. Convert it to silver sulfide and the stain appears in the highlights and the borders while the image itself may look untouched — the pattern Kodak’s toning sheet warns about in the same words, a dark yellow stain “especially noticeable in print borders and highlights”.

The historical evidence for how much silver a print can retain is worth knowing because it is startling. Spiller reported to the Photographic Society of Great Britain in 1868 that silver was retained “in the whites of the albumen print, and indeed in all parts of the coating”, and that he had found it by moistening the white surface with ammonium sulfide and watching a brown stain appear — which is the residual-silver test the next page runs, in its original form, a hundred and fifty years early. Haddon and Grundy then measured it: an unexposed albumen sheet, thoroughly fixed and washed, still held nearly 5 per cent of the silver it had been sensitised with.

That figure belongs to albumen, whose protein binds silver in a way gelatin does not, and the course does not transfer it. What does transfer is ILFORD’s modern statement of the same risk for silver-gelatin paper: work a fixer above 2 g of silver per litre and “compounds may remain in the paper base after washing and over time possibly contribute to print staining”. The mechanism is the same, the magnitude is not established, and the remedy is a fixing decision rather than a washing one.

Oxidative attack: what the air does on its own

The third route needs nothing left behind at all. It attacks a perfectly processed print.

Ag → Ag⁺ + e⁻

Oxidative attack: image silver gives up an electron to an oxidant

Once the silver is an ion it is mobile in a damp gelatin layer, and where it is reduced again decides what you see.

Ag⁺ + e⁻ → Ag

Silver mirroring: the mobile ion is reduced back to metal, at the surface

The Image Permanence Institute states the sequence plainly for a framed print: the reactions from poor framing materials, “like those we see from air pollution, are often oxidation reactions that result in image fading. In a black-and-white image the faded silver can migrate to the surface of the print and be converted back into metallic silver by other pollutants … forming a mirror-like sheen on the print’s surface.” Its glossary compresses the same thing into a definition: silver mirroring is “oxidation of black-and-white images, in which the image silver migrates to the surface, creating a mirror-like appearance”.

Three details make it identifiable rather than merely nameable.

It starts at the edges, because that is where the atmosphere reaches first. The AIC’s dry-plate entry describes a bluish metallic sheen “starting from the edges, sometimes covering the totality of the surface, and visible under reflected light”. IPI’s framing guide prints the perfect control experiment, found rather than designed: a print whose edges mirrored under a poor-quality mat while the oval the mat window left uncovered stayed clear. The mat was the source.

It is a surface deposit, and it is fragile. The AIC warns that mirrored areas are “extremely susceptible to abrasion, and rubbing in these areas may move or remove some image material”. Silver that has migrated to the surface is no longer held by the gelatin that held the grain.

Localised, it makes spots rather than a sheen. IPI defines microspots as “small colored spots, usually red or orange, caused by localized oxidation of black-and-white images”, and groups mirroring, microspots and overall discoloration together as silver image decay. The distinction between a general sheen and a scatter of coloured spots is a distinction between a general atmosphere and a point source — a fleck of something reactive in an enclosure, or a particle of the wrong dust.

⚗Which oxidants, and where they come from

The Library of Congress names them by family and by source, and the domestic ones are more surprising than the industrial ones.

Oxidant gases: nitrogen oxides, from combustion, and ozone, formed when sunlight acts on nitrogen oxide in the lower atmosphere and also “produced by some electrostatic copiers and printers”. A photocopier near a store of prints is a source, and the leaflet says so.

Acidic and sulfiding gases: sulfur dioxide and nitrogen dioxide react with atmospheric water to give sulfuric and nitric acids, which “attack all components of photographs and cause silver images to fade and paper and board supports to discolor and become brittle”.

Environmental fumes: “peroxides from untreated wood, paints, and varnishes; poor quality paper or plastic products in close proximity to photos; and the fumes from common cleaning solvents can cause images to oxidize and fade.” Which is why a new frame, a new shelf and a freshly painted room are all hazards, and why the leaflet ends up recommending water-based latex paint and chlorine-free detergents.

And one that comes out of the darkroom. Kodak’s own toning sheet records that hydrogen sulfide gas “can fog unexposed paper and film, and will oxidize unprotected silver images in negatives and prints” — which is the reason sulfide solutions are never discarded with an acid, and also the reason no photographic material is stored where sulfide work is done.

Why some images are more vulnerable than others

The chemistry above attacks a surface, so the quantity that matters is not how much silver an image contains but how much of that silver is on the outside of something.

Reilly makes the argument explicitly for printing-out papers, and it is the clearest statement of it anywhere in the corpus: the image is “composed of very small, very highly dispersed particles of metallic silver … The small particle size means that the silver has a very large surface area relative to its mass; hence a large portion of its total mass is on the surface and readily accessible to destructive chemical agents.” The Getty’s identification handbook supplies the comparison that makes it a ranking rather than an observation: photogenically formed silver particles are much smaller than chemically developed ones.

So the order of vulnerability follows particle size, and it runs roughly: printed-out silver in a salted-paper, albumen or gelatin printing-out print, most exposed; the fine silver of a slow chloride contact paper next; the coarser grain of a fast film or a bromide enlarging paper least. The middle term of that ranking is the course’s inference from the two sourced facts either side of it, not a statement any source in the corpus makes, and it is written here as an inference so that a reader can reject it.

Two structural facts sit alongside it. A developed grain is filamentary rather than compact, so it has more surface than its mass suggests, which is why negatives mirror at all. And a barrier helps: IPI records that gelatin glass plates “protected by a varnish overcoat rarely show signs of image oxidation”, which is the same argument as a resin coating or a mat that does not touch the image — put something between the silver and the air and the oxidant has further to travel.

What the standards measure, and what this course says instead

Everything above is mechanism. The question a conservator or an archive actually asks is a threshold question — how much residual thiosulfate is too much, and how is it measured — and thresholds are what the international standards carry.

This course does not hold those standards, does not quote them, and does not restate their figures. That is the owner’s ruling of 4 September 2026, applied here exactly as Part XIII applied it to speed and Part XI applied it to fixer capacity: name the standard by number, state the course’s own criterion in public where a reader can check it, and label every figure derived under it as the course’s own measurement under the course’s own criterion.

What the corpus does establish is which standards exist and what each is for, because the conservation bodies name them in documents that are free to read.

Number	What it governs	Where the course read the number
ISO 18901	The stability of processed silver-gelatin film, and the processing needed to achieve it	The Library of Congress leaflet, which tells a collection to insist that processing be done to it
ISO 18902	Albums, framing and storage materials — the enclosure standard	Two IPI guides, which give it two different titles (see below)
ISO 18916	The Photographic Activity Test for enclosure materials	IPI’s photo-safe guide and its reference list, which dates the edition to 2007
ISO 18911	Storage practices for processed safety photographic films	IPI’s reference list, 2000
ISO 18918	Storage practices for processed photographic plates	IPI’s reference list, 2000
ISO 18920	Storage practices for processed photographic reflection prints	IPI’s reference list, 2000, and the Library of Congress for the 18 °C figure it takes from it

!Three honest gaps in that table

The residual-thiosulfate test method could not be verified. The number usually cited for it is ISO 18917, and no document in this course’s corpus names it. What the corpus does carry is its American ancestor, in the Library of Congress’s own standards bibliography: ANSI PH4.8-1985, “photography (chemicals) — residual thiosulfate and other chemicals in films, plates, and papers — determination and measurement”. The course names that designation, states that it has not read it, and prints no residual-hypo limit from any standard, historical or current, because it has read none.

ISO 18902 has two titles in the corpus, both from IPI. The media storage reference lists it as “Processed photographic films, plates and papers — Filing enclosures and storage containers” (2001); the framing guide and the photo-safe guide both give “Processed imaging materials — Albums, framing and storage materials”. The most likely explanation is a retitling between editions, and the course records both rather than choosing, because it holds neither edition.

The PAT has two numbers in one IPI booklet. The Media Storage Quick Reference attributes the photographic activity test to ISO 14523 in its glossary and to ISO 18916 in its numbered references, four pages apart. ISO 14523 is the older designation. A reader who meets the earlier number on an old product label is meeting the same test.

!The Part XII washing criterion

Stated in full, because the standards that would settle it are not ones this course can quote.

A wash is complete when an unexposed white sheet of the same material, processed and washed alongside the batch, shows no colour difference between a strip immersed for about three minutes in a 1 per cent silver nitrate solution and the untreated part of the same sheet, the two compared wet, in subdued light. That is Kodak Limited’s published test, and the course adopts its endpoint unchanged.

A material is adequately fixed when a drop of ILFORD’s sodium sulfide test solution, diluted 1+9, on a washed white area leaves no more than the barely visible cream tint of a reference spot placed on a print known to be well fixed and thoroughly washed.

A print intended to last is given the maker’s optimum-permanence sequence rather than the standard one, with a non-hardening fixer, and its fixer is retired at the point Part XI’s criterion sets rather than at its commercial capacity.

What this criterion is. Two published tests with published endpoints, both executable at a sink, both read against a control carried through the same session. It is reproducible and you can check every part of it.

What it is not. It is not a measurement of residual thiosulfate in micrograms per square centimetre. It does not certify permanence, and it is nobody’s standard but this course’s. A result obtained under it is the course’s own measurement under the course’s own criterion, and the lab says so in those words.

Its known weakness, stated because a criterion that hides one is worthless: both tests are visual comparisons against a control, so their resolution is the eye’s, and neither can see the difference between a print that will last eighty years and one that will last two hundred. They detect failure, not degree.

Protective toning, in one paragraph and a forward link

If the failure is that image silver reacts, one intervention is to stop it being silver.

Kodak’s own toner publication makes the claim as broadly as it can be made: toning “converts the black-and-white silver image to an inert compound, which reduces the harmful effects of intense light, ultraviolet radiation, oxidizing gases, extremes of temperature and humidity, and fumes”, and “all Kodak toners will protect the image whether or not they produce a color shift”. The named conversions are specific: rapid selenium toner “converts the silver image to silver selenide”, brown toner converts it to silver sulfide, and gold protective solution GP-1 “provides print protection while changing the image tone only slightly”.

Two Tier 1 conservation sources say the same thing from outside the industry. The Library of Congress notes that “use of chemical toners also helps to protect silver images from deterioration”. IPI’s negatives handbook describes toning as changing the structure of the silver particles “by the addition of a more noble metal (i.e., oxidation-resistant) such as gold or the formation of a more stable silver compound (e.g., silver sulfide)”, recommended for chemical stability — and then adds the sentence that keeps this honest: “in practice this step was and is rarely performed”.

Reilly supplies the mechanism, and it is the surface-area argument again from the other side: a noble-metal toner protects “by partially replacing and enclosing” the silver with gold or platinum, metals that “react much less readily with sulfur and are much more difficult to oxidize”.

The chemistry, the choices, the dilutions and the practice are Part XX’s, and this page hands them over rather than half-teaching them. What belongs here is the reason toning appears in a chapter about permanence at all, and the fact that protective toning is the only intervention in this part that changes the image rather than what is around it.

Storage, enclosures and the conservator’s order of priorities

The environment is the other half of the race, and the numbers below are the conservation bodies’ own.

Relative humidity. The Library of Congress gives an ideal set point between 30 and 50 per cent for a mixed collection, “without cycling more than +/- 5% a day”, and 30 to 40 per cent where only photographs are stored. High RH softens the gelatin binder and makes it vulnerable; low RH shrinks and cracks it; cycling does both in turn and is worse than either. Mould “tends to grow when the RH is greater than 60% and the temperature is above 75-80° F”. IPI gives glass plates the narrower band of 30 to 40 per cent.

Temperature. The highest recommended extended-term storage temperature for black-and-white prints and negatives on polyester base is 18 °C, with daily fluctuations greater than ±2 °C avoided — the Library of Congress citing ISO 18920, which defines “extended-term” as “when it is desired to preserve information for as long as possible”. Colder is better if the humidity is also low, and cold storage has costs in money and in access that the leaflet is candid about.

Enclosures. The rule is one test and one standard. The Photographic Activity Test, ISO 18916, puts the material in prolonged contact with two detectors — one screening for oxidation and reduction reactions “which can cause image fade, silver mirroring, and red or gold spots”, one for chromophores that yellow the support. ISO 18902 then adds requirements the PAT does not cover: a pH not below the reference water and below 10, an alkali reserve of at least 2 per cent calcium carbonate, a lignin content low enough to give a Kappa number of 7 or below, and a colorant-bleed test. IPI is explicit that a material passing “only the pH requirements or only the PAT” is not necessarily photo-safe, and that photo-safe “refers only to the chemical reactivity of a material” and says nothing about abrasion, creases or tears.

Framing. ISO 18902 recommends glazing that blocks at least 97 per cent of ultraviolet energy. And IPI’s warning about the words on the packet is worth memorising: “acid-free”, “archival”, “museum-quality” and “conservation board” are “not standardized or legal terms but are simply marketing terms”. The claim that means something is the printed sentence naming the standard.

What stands between an oxidant and a grain of image silver

an oxidant arriving from the room, working inward

1. The roomtemperature, relative humidity and what is in the air. It sets the rate of every reaction below it, and it is the layer the conservation bodies say to fix firstlargest effect
2. The enclosuresleeve, mat, box or frame. It can protect, and if it fails the tests it becomes a source rather than a barriercan be a source
3. The binder and any coatinggelatin, a resin coating, or a varnish. IPI records that varnished plates rarely show image oxidationfixed at coating
4. The image silver itselfits particle size decides how much of it is surface, and toning decides what an oxidant finds when it arrivestoning acts here
5. What you left in itresidual thiosulfate and residual silver, which need no arrival at all: they are already insidewashing and fixing act here

Five layers, in the order an attack from outside meets them, with the two the darkroom controls at the bottom. The ordering of effect is the Image Permanence Institute's own: improving the climate does more than improving the enclosure, and neither can undo what a short wash left behind.

✦The conservator's priority, which is not the one most photographers guess

IPI puts it in two sentences that reorder everything above: “enclosures cannot overcome deficiencies in the storage climate”, and “while providing new, more inert enclosures is a desirable aspect of preservation practice, improving the climate is more effective overall.”

So the order is: wash and fix properly, because that is free and nothing later can undo it; then get the prints out of the loft and the garage into a stable interior room; then buy better enclosures; then think about toning. Most people buy the boxes first.

Deciding how permanent a print has to be

ILFORD draws the line itself, and the wording repays attention: “the standard fixing and washing recommendations will give excellent print permanence for all commercial needs. When optimum permanence is needed, perhaps for archival storage of prints, the following … sequences are recommended.” Two named tiers from the manufacturer, with a different procedure for each, and the choice handed to you.

The choice is real because the costs are real, and they are not only in time.

The optimum-permanence sequence costs ten minutes in a wash aid, a non-hardening fixer, and the discipline of retiring the bath early. It buys the largest improvement per unit of effort available anywhere on this page, and it costs less water than the sequence it replaces.

Protective toning costs a tone change you may not want, a chemical you would not otherwise own, and — for the sulfide route — a hazard the course treats carefully. It also costs the print’s neutrality, which for some work is the whole point of the print.

Cold storage costs money, and access: material has to equilibrate before it can be handled.

And a print you can remake costs nothing to lose. A work print pinned to a wall in daylight is doing its job; a print sold, given away or accessioned cannot be remade by its owner. It is reasonable to process the second differently from the first, and unreasonable to process both to the standard of the more demanding one and then wonder why printing has become slow.

What is not a legitimate choice is not knowing which you made. The test in the next page takes twenty minutes and settles it.

Recap

• Two clocks. Residual chemistry is finite and yours; environmental attack never stops. The second decides how fast the first acts.
• Sulfiding converts image silver to silver sulfide, using labile sulfur from residual fixer or from the air, and shows as a general yellowing with density lost.
• Residual silver is distributed with the unexposed halide, so its stain appears in the highlights and borders while the image may look untouched. It is a fixing fault, not a washing one.
• Oxidative attack takes an electron from image silver; the mobile ion migrates and is reduced again at the surface as silver mirroring, or locally as coloured microspots. It starts at the edges, and a bad mat or frame is a common source.
• Finely divided silver is the vulnerable kind, which is why printed-out images fade first and why toning, which replaces or encloses the silver, protects.
• The standards are named by number and not quoted, because the course does not hold them. The residual-thiosulfate method standard could not be verified at all; the course prints no limit from any standard.
• The Part XII washing criterion is two published visual tests read against controls carried through the same session — reproducible, checkable, and honest about detecting failure rather than degree.
• Climate before enclosures. IPI’s own priority: a better box cannot fix a bad room.

Check your understanding

Question 1. A fibre-base print from the 1970s shows clean, near-white borders, a normal-looking image, and a bluish metallic sheen along all four edges that is strongest where a mat once sat. What failed, and when?

• Washing failed at the time of processing, leaving thiosulfate that has since sulfided the image
• Fixing failed at the time of processing, leaving residual silver that has stained the whites
• Nothing failed in processing; an oxidant reached the print in storage, and the edges went first because that is where it arrived
• The paper base has become acidic and is attacking the emulsion from below

Show the answer and why

Answer: Nothing failed in processing; an oxidant reached the print in storage, and the edges went first because that is where it arrived

Read the distribution, not the damage. Sulfiding from residual thiosulfate is general, because the thiosulfate was general. Residual silver stains the highlights and borders, because that is where the unexposed halide was. A metallic sheen concentrated at the edges and following the shape of a mat is neither: it is oxidation of image silver, migration of the mobile ion to the surface and reduction back to metal there, with the atmosphere and the mat as the sources. IPI publishes exactly this case, a print whose covered edges mirrored while the oval the mat window left open stayed clear.

Question 2. Two prints from the same session are stored together for thirty years. One was washed to the criterion on this page and one for two minutes. Both are untoned, in the same damp frames. What do you expect, and why is the answer not simply "the badly washed one is worse"?

• Only the badly washed print changes; the well-washed one is permanent
• Both change, but by different routes: the badly washed print yellows generally from its own thiosulfate as well as taking whatever the atmosphere brings, while the well-washed one shows only the environmental damage
• Both change identically, because storage dominates over processing
• Neither changes, because thirty years is too short for either mechanism

Show the answer and why

Answer: Both change, but by different routes: the badly washed print yellows generally from its own thiosulfate as well as taking whatever the atmosphere brings, while the well-washed one shows only the environmental damage

Permanence is a race between two clocks and washing only stops one of them. The well-washed print still has image silver exposed to oxidant gases, peroxides off the frame and moisture, so it can still mirror and still fade; what it cannot do is sulfide from a supply it is carrying. The badly washed print suffers both, and Reilly notes that the onset of yellowing depends on the storage environment and the residue together rather than on either alone, so the damp frame accelerates the failure the short wash made possible. This is also why toning and storage are separate interventions from washing rather than substitutes for it.

Question 3. Why does this page name ISO 18901 and ISO 18916 but print no residual-thiosulfate limit from any standard?

• Because the limits differ between film and paper and no single figure would be correct
• Because the course does not hold the standards, quotes no part of them, and could not even verify the number of the residual-thiosulfate method standard against any document it has read
• Because residual thiosulfate cannot be measured outside a laboratory
• Because the historical ANSI limits have been superseded and the modern ones are secret

Show the answer and why

Answer: Because the course does not hold the standards, quotes no part of them, and could not even verify the number of the residual-thiosulfate method standard against any document it has read

The ruling that governs it says the course states its own convention explicitly, cites each standard by number as the thing the convention is modelled on, and never prints a threshold as though quoted. The numbers in the table were all read from free conservation publications that name them; the residual-thiosulfate method standard was not named in any of them, so the course records the gap, names its American ancestor ANSI PH4.8-1985 as the designation it did find, and states that it has read neither. A published criterion a reader can execute beats an appeal to a document neither of you can read.

Question 4. Kodak states that all its toners protect the image whether or not they change its colour. What is the mechanism, and what does it imply about which images benefit most?

• The toner deposits a physical barrier over the whole print surface, so a thicker coating protects better
• The toner converts image silver into a less reactive compound or encloses it in a more noble metal, so the images that benefit most are those with the greatest silver surface area per unit mass
• The toner removes the last of the residual thiosulfate, so the images that benefit most are the badly washed ones
• The toner hardens the gelatin, so the images that benefit most are those on unhardened paper

Show the answer and why

Answer: The toner converts image silver into a less reactive compound or encloses it in a more noble metal, so the images that benefit most are those with the greatest silver surface area per unit mass

Kodak names the conversions: selenium to silver selenide, brown toner to silver sulfide, gold as a protective plating. Reilly gives the reason it works and the reason it matters most for fine particles: the silver of a printed-out image has a very large surface area relative to its mass, so a large part of it is on the surface and accessible to sulfur and oxidants, and gold and platinum react far less readily with both. Toning is not a substitute for washing and does not remove thiosulfate; it changes what the attacker finds when it arrives.

Question 5. A supplier sells storage sleeves labelled "acid-free, archival quality". A second sells sleeves whose packet reads "Passes ISO 18916". Which claim carries more information, and what does the better one still not cover?

• The first, because acid-free covers pH and archival covers everything else
• The second, because it names a test; but the PAT alone does not make a material photo-safe, since ISO 18902 also requires pH limits, an alkali reserve, a low lignin content and a colorant-bleed test
• Neither, because both are marketing claims with no standing
• The second, and it is complete: passing the PAT is the definition of photo-safe

Show the answer and why

Answer: The second, because it names a test; but the PAT alone does not make a material photo-safe, since ISO 18902 also requires pH limits, an alkali reserve, a low lignin content and a colorant-bleed test

IPI states that "acid-free", "archival", "museum-quality" and "conservation board" are marketing terms that are neither standardised nor legal, so the first claim is unfalsifiable. The second names ISO 18916, the photographic activity test, which screens for oxidising and reducing agents and for chromophores. But IPI is equally explicit that materials passing only the PAT are not necessarily photo-safe, and that photo-safe refers only to chemical reactivity and not to abrasion, creases or tears. The claim worth looking for is the one naming ISO 18902, which subsumes the PAT and adds the rest.

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