IPI Storage Guide For Acetate Film

lTHE WHEELAverage RelativeHumiditySIDE TWOApproximate Numberof YearsAverageTemperatureFig. 2The WheelSIDE ONEThe Wheel presents the research results about vinegarsyndrome in a format that makes it easy to evaluate theeffect of a particular storage environment on the lifeexpectancy of acetate film. It makes it possible tocompare the rate of deterioration at a wide range ofconditions, to determine which of several existingstorage environments is better for film, or to considerthe various combinations of temperature and RH thatwould yield a specified life expectancy. The Wheel, asnoted, has two sides (see Fig. 2). Side One is based onthe approximate number of years it would take fresh(new) acetate film to start degrading (to reach 0.5acidity see p. 14) at various storage conditions. SideTwo is based on the approximate number of years itwould take partially degraded film (film that hasalready reached 0.5 acidity) at various storage conditions to reach an acidity level of 1.0. Printed on eachside, around the circumference of the larger disk, is arange of temperatures from 30 F/-1 C to 120 F/49 C.Beside the window on Side One of the Wheel is printeda range of humidity levels from 20% to 80% RH in 10%increments. Side Two has only three increments: 20%,50%, and 80% RH. The predicted number of years ateach temperature/humidity combination is displayed inthe window on each side.4Evaluating a Storage ConditionSuppose, for example, that a collection of fresh acetatefilm is kept in an air-conditioned room where it scomfortable for people: 70 F (21 C), with an RH of50%. (We ll assume these conditions are maintainedyear round not always an easy thing to do.) Thequestion is, how good are these conditions for the film?To find the answer, consult Side One of the Wheel (theblue side). Rotate the smaller disk until the arrow pointsto the temperature in question in this case, 70 F. Acolumn of numbers will appear in the window. Next,select the RH of interest in this case, 50%. Readingacross on the same line, we find in the window thepredicted number of years for that set of storageconditions in this case, 40. (See Fig. 3)At each temperature on Side One, the predictednumber of years until vinegar syndrome begins to be aproblem are shown for seven different humidity values.Notice that lower RHs (dryer conditions) have longerpredicted times, while higher RHs (more humidconditions) have shorter predicted times. This is alwaystrue, regardless of temperature, because the roomhumidity controls how much water will be absorbed bythe film. The water content of the film plays an essentialrole in the reactions of deterioration; therefore, thehigher the RH, the faster the degradation.Note in Fig. 3 the difference between the predictions at 50% RH and 20% RH. Generally, film degradesonly about one third to one half as fast at 20% as it doesat 50% RH. It s not a good idea to go lower than 20%RH, though, because film dries out too much andbecomes brittle. Likewise, it s not a good idea to storefilm at RHs above 50%. As Fig. 3 shows, the predictedtime at 70 F, 80% RH is considerably shorter than thatat 70 F, 50% RH. In addition, high humidity not onlypromotes vinegar syndrome, it also can allow mold togrow (if RH is kept at 70% RH or above for sustainedperiods), leading to irreversible damage to the gelatinemulsion.The prediction for room temperature (70 F/21 C)and 50% RH in our example may seem surprisinglyshort only about 40 years. Nearly every film archivistknows of film that is 40 years old and still in goodcondition. (And many archivists also know of 40-yearold film that is not so good.) To understand the 40-yearprediction, remember that it is the approximate numberof years to the onset of measurable deterioration, andnot the number of years it would take for film to

50% 40 Yrs.Fig. 3Detail of SideOne of theWheel set for70 F/21 C.reach the later stages of deterioration, when it wouldbecome shrunken and brittle. Remember also that thepredictions assume maximum acid trapping and thatreal-life films have usually experienced a changingenvironment, not a steady one. In spite of all thesequalifications, and although the evidence is anecdotaland not rigorous, there does exist enough actual keepingexperience with acetate film to confirm the generaltrends in the Guide. Film kept in warmer than roomtemperature conditions does degrade in about 30 years,and film kept at cooler than room-temperature storagefor 50 years shows no signs of degradation. The generaltrends in the IPI data have been confirmed by similarlaboratory studies at Kodak4,5,6 and Manchester Polytechnic in the U.K.7,8,9,10 So, while the predictions shouldnot be taken literally, neither are they unrealistic.However, their importance is that they can be relied onto quantify in a relative manner how much better orworse one storage condition is than another.allow the use of the Guide to determine which storageconditions would provide the desired number of yearsbefore the onset of vinegar syndrome. In our examplewe will assume that the institution wants its acetate filmto last a minimum of 100 years.Is Room Temperature Good Enough?If all the collection materials must coexist in onestorage room, then because of the possible problemswith glass plates at RHs lower than 25%11 (see p. 19),an RH of 40% is decided upon. But what should thetemperature be? The most convenient temperature isprobably a comfortable room temperature of 72 F(22 C). Using Side One of the Wheel, find the temperature closest to 72 in this case, it s 70 F (shown inFig. 3). Although this is slightly cooler than the 72 Fwe re interested in, it s close enough. At 40% RH, 70 F,the predicted years to onset of vinegar syndrome is 50 only about half what the institution wants from theirfilm. Clearly, room temperature is not a good choice tomeet the preservation objectives of the institution, andsomewhat cooler temperatures are required.What Are the Choices?To determine which temperature would yield thedesired 100 years at 40% RH without danger of vinegarsyndrome (remember the predictions on Side One of theWheel are for fresh, undeteriorated film), rotate thesmaller disk of the Wheel until 100 years or moreappears in the window opposite 40% RH. At 60 F wefind the desired 100 years (Fig. 4, left). If the collectioncontains older films with a questionable storage history,leading the collection manager to suspect that deterioration is already starting (even though obviousUsing the Wheel to Plan a New Film StorageEnvironmentThe Wheel is also useful for planning a new filmstorage environment. For example, let s assume aninstitution has the opportunity to build a new storagearea for its photographic collection one that consistsof a variety of materials, including film, glass plates,color and black-and-white prints, etc. Since the collection has sizeable amounts of acetate film in the form ofblack-and-white sheet films and color transparencies,prevention of vinegar syndrome is one of several goalsfor the new storage area.The first thing to do is to decide how long theinstitution wishes to retain its film collections. This willFig. 4 Details of Side One of the Wheel set for60 F (left) and 55 F (right).5

used to slow the progress of deterioration, and one canget a good idea of that from the predictions of timenecessary for free acidity to go from 0.5 to 1.0.Fig. 5 Details of Side Two of the Wheel set for60 F (left) and 40 F (right).outward signs of it are not apparent), then somewhatcooler temperatures might be chosen to provide a little cushion on the 100 years. At 55 F, 40% RH, forexample, the prediction for onset of vinegar syndromefor fresh film is 150 years (Fig. 4, right).However, if there is evidence that film in a collection is indeed already deteriorating (odors, waviness onthe edges of the film, etc.), then the situation is different. Can improved storage stabilize such film so thatit may be kept around for a significant number of years?The answer is a qualified yes, but colder temperaturesare required to achieve a long life.Planning for Degrading FilmSide Two of the Wheel deals with storage conditions foralready degrading film. The predictions on this side ofthe Wheel are based on the period of years necessary forthe free acidity of film to double from 0.5 to 1.0. Thesenumbers take a little explaining. (For more information,see p. 13 for a discussion of the course of the vinegarsyndrome.) At the 0.5 free acidity level, film may smellof acetic acid, but it is still quite usable. At 1.0 freeacidity the same is true, but the odor may be a littlestronger; not until the free acidity reaches about 5.0 willchanges (buckling, embrittlement) occur to render thefilm unusable.All this is just to explain that the doubling ofacidity to 1.0 doesn t mean that terrible things suddenlyoccur it only indicates that deterioration has progressed. The acidity has doubled, but there is still someway to go before the film is ruined and useless. Theimportant question is how storage conditions can be6Returning to our example of choosing storageconditions for a mixed collection, if some of the acetatefilm were already degrading, Side Two of the Wheelcould be used to determine how the 60 F, 40% RHcondition would affect the progression of deteriorationin such film. Rotate the smaller disk on Side Two topoint to 60 F, and locate the nearest RH value to the leftof the window (Fig. 5, left). Side Two has only threeRH values (20%, 50%, and 80% RH), so in this case wewill have to interpolate between the 20% and 50%values. At 60 F, 50% RH the free acidity will double inten years, at 20% RH, it will double in 45 years. Since40% RH is closer to the 50% value, it will take about 20to 25 years for free acidity to double in films kept at60 F, 40% RH.This is encouraging, but suppose the institutiondesired 100 years of useful life from already degradingfilm. (Remember that once channeled or embrittled,nothing can be done to reverse the course of deterioration prevention is the only effective method ofpreservation.) Rotating the smaller disk on Side Two to40 F shows that, at 50% RH, 50 years is predicted beforefree acidity doubles from 0.5 to 1.0 at that low temperature condition (Fig. 5, right). Interpolating to 40% RH,the prediction is for approximately 100 years. Thus theinstitution can reasonably expect that storage at 40 F,40% RH would provide a century of additional usefullife, even for film that was already in the initial stages ofdegradation (at an acidity level of 0.5).Color FilmOf course, vinegar syndrome is only one considerationin the choice of a storage environment for a photographic collection. In our hypothetical example, colorfilm is also included in the collection. Dye fading alsohas a strong temperature and RH dependence, but no wheel yet exists for color. A decision to choosetemperatures cooler than room temperature will alsobenefit the color film and prints in the collection.12Many real-life institutions have opted for cold storagefacilities to forestall simultaneously both dye fading andacetate base deterioration.

lTIME CONTOURS FOR VINEGAR SYNDROMEThe Time Contours for Vinegar Syndrome (representedtwice for Fahrenheit and Celsius) give a broadoverview of the relationship between temperature, RH,and the time in years for fresh film to begin deteriorating. (See Fig. 6.) Each sloping line on the graph ismarked with a time period (one year, five years, etc.);any point on the line represents a temperature/RHcombination for which the indicated life expectancywould be obtained. About 100 years of life expectancy,for example, could be obtained at 70 F, 20% RH, or at48 F, 80% RH, or at any of a variety of intermediatecombinations. This graph makes it possible to see thegeneral trends at a glance such as how short film lifecan be if stored in hot and humid places, and how longit can be kept under cold, dry conditions (15 centuries at30 F, 20% RH).It is important to realize that these time contoursare useful for a quick approximation of the relationshipsFig. 6between temperature, humidity, and the onset of thevinegar syndrome. Don t try to read between thelines. On this type of graph, interpolation between thelines is difficult because the time intervals are notuniform; to find a life expectancy associated with aparticular set of conditions, use the Wheel or the TimeOut of Storage Table instead.The Time Contours show the extreme range of lifeexpectancies that are possible with acetate film. Theyrange from very short (only one year at 130 F, 50%RH) to very long (more than 1000 years at 30 F, 50%RH). Even longer life expectancies are possible at stilllower temperatures, but time periods of more than tencenturies seem a bit unreal to us even though manyobjects survive that are three or four millennia old. Thedownward sloping lines of the Time Contours graphshow how RH affects film life; at higher RHs the lifeexpectancy of film is much shorter.Time Contours for Vinegar Syndrome7

lTHE TIME OUT OF STORAGE TABLEEven if a collection has a special environment such as acold storage vault, objects do not always remain there.They may be brought out periodically for use, formaintenance of the vault, or for other reasons. Makinguse of an approach first suggested by Mark McCormickGoodhart, a research scientist at the Smithsonian'sConservation Analytical Laboratory, the Time Out ofStorage Table (Fig. 7) gives data similar to that foundon the Wheel, but with the added dimension of timeout of storage to show how film life expectancy isaffected when film is taken from a storage vault andused at room temperature for a period of time. Theeffects of leaving the storage environment and spendingvarying amounts of time at office conditions (75 F, 60%RH) can be profound. Film life expectancy can besignificantly diminished merely by removing film fromspecial storage and keeping it for an average of 30 dayseach year at office conditions.Using the TableTo use the Time Out of Storage Table, one must firstlocate the row that corresponds to the vault conditions. The Table is based on theassumption that, most of the time,film is stored in a primary storagearea (referred to as the vault, though it could be an ordinary room,a special vault, a refrigerator, or anyother kind of physical storagearrangement). What matters is thetemperature and RH of this primarystorage or vault condition wherefilm normally resides. The three lefthand columns of the Time Out ofStorage Table are marked PrimaryStorage or Vault Conditions. Thefirst two columns are temperature(shown for convenience in bothFahrenheit and Celsius), the third isRH.Using the Time Out of StorageTable involves finding the rowcorresponding to the primary storagecondition and then looking acrossthe table at columns that refer to howmuch time the film is out of the vaultand kept in an office condition of75 F (24 C), 60% RH. This is meantto reflect the reality that film is oftenremoved from a vault for use, forexhibition, for curatorial research,because of equipment failure, formaintenance, etc.Each column heading is markedwith the average number of days peryear that film might spend out of thevault and in the office conditions.Thus the 120 Days column meansthat film spends four months eachFig. 78Time Out of Storage Table.

year in the office condition and eight months in thevault condition. The numbers in the columns are thepredicted number of years for the onset of vinegarsyndrome (defined as reaching 0.5 free acidity) in freshacetate film; the predictions show the combined effectsof being stored part of the time in the vault conditionand part of the time in the office condition.Consider an example of a vault that is operating at40 F, 40% RH (Fig. 8). This is an excellent storagecondition that should provide a very long life for films.If film were always kept in the vault, then the predictedtime before the onset of vinegar syndrome in fresh filmwould be 450 years. The column in the Time Out ofStorage Table marked 0 Days repeats the samepredictions as Side One of the Wheel this simplymeans the film never leaves the vault. Looking acrossthe row, the predicted number of years drops off sharplyfrom 450 down to 60 as the average number of days peryear out of the vault increases.At 30 days per year out of the vault, the predictedtime is only 175 years, less than half of the value for 0days. It isn t difficult to imagine circumstances in whichfilm would average 30 days per year out of a coldstorage vault, considering all the possible reasons forintentional or unintentional removal or equipmentFig. 8shutdown. If the film is out an average of 120 days peryear, the predicted time is only 60 years, about 13% ofthe life expectancy if it never left the vault.What Happens to Film Out of Storage?The longer the time out of the vault, the more thewarmer condition takes over in determining theoverall life expectancy of the film. Note that in readingdown the column marked 120 Days out of storage(Fig. 7), there is little or no benefit from having a filmvault any colder than 50 F. The time spent at the office condition is determining the life expectancy ofthe film. No matter how much colder than 50 F thevault condition may be, the four months per year at75 F, 60% RH dictates that only 60 years will passbefore the predicted onset of vinegar syndrome.The impact on film life expectancy of the combinedeffects of a storage vault and an office (i.e., use)condition will of course depend on the temperature andRH of both areas. The Time Out of Storage Table ispredicated on an office condition of 75 F (24 C), 60%RH. The predictions in the Table will be different forother office conditions. IPI can, if desired, calculate acustomized Time Out of Storage Table based on adifferent use environment; institutions can have thisdone on a consulting basis by contacting IPI.Detail of the Time Out of Storage Table showing a primary storage condition of 40 F.9

l CHEMICAL DETERIORATION OF FILM BASESFig. 9Ruined gelatinin degraded35mm nitratemotion picturefilm.Chemical Deterioration of NitrateMany archivists are familiar with the slow chemicaldeterioration of cellulose nitrate film,13,14 but they haveassumed that cellulose acetate (so-called safety film)is something entirely different. It turns out that cellulosenitrate and cellulose acetate both share a built-inpropensity to degrade; both these plastic materials aremodified forms of cellulose, and both have a regrettabletendency to become un-modified by the same kinds ofchemical pathways. To make nitrate plastic in the firstplace, nitro (NO2) groups are grafted onto the longmolecular chains of native cellulose.15As long as these side groups stay put, all is well.But in the presence of moisture, acids, and heat theytend to become detached, liberating the nitro groups.16The nitro groups are very harmful substances to letloose they are strongly acidic and strongly oxidizingin nature. They fade silver images and make gelatin softand sticky; their highly acidic fumes rust film cans andembrittle paper enclosures.17 This, however, is nitrate a sad tale, but one already told many times. How doesacetate degradation differ from nitrate degradation, andwhere does the vinegar syndrome come in?Acetate Film and Vinegar SyndromeVinegar syndrome is a problem that affects onlycellulose acetate plastic materials.18 In acetate film the side groups are not nitro (NO2) but acetyl (CH3CO see Fig. 10). As with nitrate, all is well as long as theseacetyl groups remain grafted onto the cellulose mol-ACETYL GROUPSFig. 10 A portion of a cellulose acetatechain. Two units shown; molecule goes on.10ecules. Detachment of the acetyl groups also can occurin the presence of moisture, heat, and acids only, inthis case, free acetic acid is liberated.The acetic acid is released inside the plastic, but itgradually diffuses to the surface, causing a familiar sharpodor the odor of vinegar (which in fact is a 5% aceticacid solution in water). The amount of acetic acid thatcan be generated by degrading film is surprisingly large;expressing it in terms of teaspoonfuls of householdvinegar, in advanced stages of deterioration there can benine teaspoonfuls or more of vinegar for every four feetof 35mm movie film. A typical 1000-ft. can of 35mmfilm can generate enough acetic acid to be equivalent to250 teaspoonfuls of household vinegar!So, to restate, cellulose nitrate film and celluloseacetate film are somewhat similar in that they are bothmodified forms of cellulose, and both can slowlydecompose under the influence of heat, moisture, andacids. Not surprisingly, the condition

most real-life film storage is more likely to be closed than open. Motion picture film is typically stored in closed cans, and sheet films are often kept in close quarters in a box or drawer. Though the fiacid-trapping factorfl can certainly make a big difference in how long film will last, if you evaluate film storage with the