Caring for plastics and rubbers

Recommendations

• Do not store objects or packages directly on floors.
• Use protective waterproof covers in storage and display, assuming that all pipes will leak and all sprinklers will activate.
• Use waterproof storage containers, such as PE bags and PE and PP boxes (except malignant plastics).
• Avoid freezing wet plastics (do not freeze-dry).

For more information on mitigating risks from water, consult Agent of deterioration: water.

The flammability of cellulose nitrate

CN is of particular concern because it is the most flammable plastic and it generates oxidizing gases as it burns, making the fire impossible to extinguish by depriving it of oxygen. Fire suppression sprinklers and misters, foam-producing nozzles, pre-charged carbon dioxide and Freon substitute dump systems have all been shown to be ineffective for CN motion picture fires. Fires in motion picture archives have been extinguished with massive amounts of water, which is thought to have cooled the burning CN below the ignition temperature, thus extinguishing the fire (Vitale 2009).

CN is made from cellulose reacting with nitric acid, which replaces some hydroxyl groups on the cellulose with nitrate groups. The extent of replacement (degree of substitution) is measured by the nitrogen content. Products with different nitrogen contents are used for different purposes, for example, plastics and lacquers contain 10.5-11.5% nitrogen; lacquers, films and film sheets contain 11.5-12.3% nitrogen; and explosives and gun powders contain 12.4-13.5% nitrogen. Fire risk increases with nitrogen content. Motion picture film has higher nitrogen content than photographic sheet film, which in turn has higher nitrogen content than plastics and lacquers.

Serious fires in motion picture archives have been attributed to spontaneous combustion of degraded motion picture films. No fires have been attributed to spontaneous combustion of sheet film stored in paper sleeves or of plastic objects. The fire hazard of CN objects is similar to that of other flammable materials such as gasoline and solvents.

CN objects should be stored in special regulated storage, preferably at low temperatures in refrigerators or freezers. In addition to reducing risk from a CN fire, segregation reduces risk of exposure to the rest of the collection to harmful oxidizing and acidic volatile degradation products.

CN objects should not be sealed within unventilated enclosures, such as bags and cabinets. In these conditions, degradation gases accumulate around the plastic in the enclosure and increase the rate of degradation, thereby increasing fire risk by decreasing the ignition temperature of the plastic. If sealed storage is required, such as to control relative humidity (RH) in a bag, then sorbents should be included in the bag to decrease the concentration of degradation gases in the bag.

Recommendations

• Treat risk of fire for most plastics (except CN) as equivalent to or less than natural organic materials like paper, natural fibres and wood (Class A).
• Burning plastics (except CN) can be extinguished by methods suitable for Class A fires.
• Segregate CN from general collection.
• CN should be stored in cold storage in appropriate freezers (small collection) or refrigerated rooms (large collections) to reduce degradation, preserve objects and reduce fire risk.
• Large collections of CN are preferably stored in a separate, specialized storeroom, built according to relevant codes.

Additional information for the display and storage of museum objects containing CN can be found in CCI Note 15/3 Display and Storage of Museum Objects Containing Cellulose Nitrate.

Recommendations

• Treat plastics as more fragile than expected.
• Avoid direct handling of objects
  • Use trays and containers.
• Always support from below.
  • Never handle by appendages or other features that jut out.
• Use rigid padded and shaped supports.
• Avoid flexing and vibration.
• Check for cracks and crazing, especially at glued joints, in painted areas and under or around adhesive labels, then support as required.
• Be wary of creep in objects and storage materials.
• Avoid contact with those solvents and solvent vapours that promote ESC.
• Avoid abrasive and chemical cleaners.
• Do not attach adhesive labels.

For more information on preventive conservation strategies to control physical forces, consult Agent of deterioration: physical forces.

Recommendations

• Install good dust control.
• Check reused plastic products for absorbed pollutants, especially acids.
• Identify and monitor malignant plastics for emissions.
• Store objects at lowest temperature possible, to decrease degradation reactions.
• Block oxidation for objects susceptible to oxidation, such as rubber and PUR, by anoxic storage using bags made of flexible plastic oxygen barrier films, inert gas atmospheres, and oxygen absorbers. Anoxic storage is only suitable for objects that degrade by oxidation.
• Do not to clean surface accretions and dirt unless the object is to go on display or it is certain that accretions are causing degradation.
• If necessary, clean objects under dry conditions by careful brushing, vacuum cleaning, or dry wiping, taking care to avoid scratching by abrasive particles and action of solvents.
  • Never use solvents, bleaches, detergents, wax or vinyl and rubber erasers.
  • If water is required, use only moist wiping.
• Use non-stick shields and interleaves made of non-fibrous and non-foam sheets, such as PTFE, PET or SI, between sticky surfaces and objects to prevent staining by contact with surfaces having plasticizer or other additive exudation (bloom).
• Monitor objects in display and storage areas for emission of acids using indicator papers and ribbons with cresol red for nitric acid from CN and bromocresol green for acetic acid from CA (refer to Use and formulation for cresol red and modified bromocresol green indicators).

For further information on effects and mitigation of pollutants, consult Agent of deterioration: pollutants.

Recommendations

• Treat risk of light and UV damage to plastics as similar to natural organic materials, such as paper, wood, natural fibres and leather.
• Avoid all exposure to UV and sunlight.
• Use minimum intensity of light during display.
• Store objects in the dark.

For more information on damage caused by light, UV and infrared, as well as control strategies to prevent damage by them, consult Agent of deterioration: light, ultraviolet and infrared.

The effect of increased temperature

Chemical degradation of plastics occurs mainly by oxidation (reaction with atmospheric oxygen) and hydrolysis (reaction with water contained in the plastic). Plastics have different sensitivities to oxidation at room temperature, as shown in Table 5.

When temperature is increased, the rate of oxidation and hydrolysis increases, and thus the rate of chemical deterioration increases. The effect on plastics is about the same as on natural organic materials like wood and leather. The rate of deterioration approximately doubles for each 5ºC increase or decreases by half for a corresponding decrease in temperature. Decreasing temperature is an effective control strategy for these degradation reactions, and is the current recommended practice for the storage of CN objects, and CN and CA photographic film (Bigelow 2004). Those plastics listed in Table 5 with the highest sensitivity to oxidation will be damaged most by increased temperature in storage, and conversely will benefit most by storage at decreased temperature.

Temperature and physical properties

Physical and structural properties such as softness, flexibility, creep and strength change when temperature changes. Amorphous thermoplastics are particularly sensitive to temperature and are more likely to deform under stress at higher temperatures. This tendency is enhanced in the presence of organic solvents.

Measures of the sensitivity of plastics to increased temperature are:

• their glass transition temperature (T_g), the temperature at which the plastic changes from a stiff glassy material (below T_g) to a flexible rubbery material (above T_g)
• their heat deflection temperature (HDT), the temperature at which a bar of plastic measuring 127 mm (5 in.) in length, 13 mm (1⁄2 in.) in depth by any width from 3 mm (1⁄8 in.) to 13 mm (1⁄2 in.), supported at points separated by 101.6 mm (4 in.), becomes sufficiently flexible to deflect (bend) by 0.25 mm (0.01 in.) when subjected to a load of 1.82 Mpa (264 lb/in.²) at its centre

The greater the T_g or HDT of a plastic, the less likely the plastic is to deform as temperature is raised.

Ranges of interest for increased temperature are:

• 15 to 30°C in an occupied room
• 30 to 45°C in an unoccupied storage room without heating, ventilation or air conditioning
• above 45°C during heat treatments

Sensitivity of physical properties of plastics to increasing temperature ranked according to T_g and HDT is shown in Table 6.

The effect of decreased temperature

As temperature is decreased, some plastics change from tough flexible materials to stiffer more brittle materials, especially if they are cooled below their T_g. Plastics shrink as they are cooled. Unrestrained plastics will not shatter spontaneously if they are cooled, but they will be more brittle and must be handled and supported accordingly, and not subjected to shock and vibration. Contraction of restrained plastic or plastic components in composite objects can result in fracture.

Measures of sensitivity of plastics to decreased temperature are T_g and brittleness temperature (T_brittle), the temperature at which a plastic fails upon impact under specified conditions. Below this temperature, the plastic fractures. Above this temperature, no fracture occurs.

Ranges of interest for decreased temperature are:

• 15 to 4°C for objects moved from room temperature to cool storage in refrigerators
• 4 to -20°C for objects moved from cool to cold storage in freezers
• <-20°C for objects stored outdoors or in unheated storage rooms below freezer temperature

Sensitivity of physical properties of plastics to decreasing temperature ranked according to T_g and T_brittle is shown in Table 7.

When to use cool (4ºC in refrigerator) or cold (-18ºC in chest freezer) storage

Cool or cold storage should only be used for those plastics for which the benefit of reduced rates of oxidation and hydrolysis outweigh the risks of physical damage due to loss of flexibility and shrinkage-induced stresses, such as cellulose esters, PUR foam, natural rubber, and perhaps polyolefins. In all cases, objects should be supported and cushioned with inert materials to prevent objects from flexing.

Objects going into cool and cold storage should be bagged, preferably with moisture buffering material (e.g. matboard), with the bag collapsed so as to contain a minimum amount of air space. Bags and moisture buffers prevent damage from condensation when objects are moved from cold to warm conditions. Methods of implementing cool and cold storage are discussed below in the section Control strategies for mixed collections containing malignant plastics.

To decrease hydrolysis, temperature and RH should be reduced. Since storage at cool (refrigerator) or cold (freezer) temperatures requires equipment that is expensive to buy and maintain, it may be more beneficial and cost effective to use dry storage at RH less than 10% without refrigeration. This can be easily and inexpensively achieved by double bagging with dry desiccants and RH sensor strips to monitor RH. According to Michalski (2000), storage at 20ºC and 8% RH increases the lifetime of low stability objects by a factor of 10, from 30–100 years (at 20ºC and 50% RH) to 300–1000 years. With some cooling, the increase in lifetime will be even greater.

Shashoua (2004) noted that decreased temperature reduces the loss of volatile components, such as plasticizers, so highly plasticized plastics, such as flexible PVC, benefit from cold or cool storage.

Temperature fluctuation

The coefficients of thermal expansion (CTE) of plastics are about 10 times greater than those of metals and wood, and 100 times those of ceramics. Temperature changes cause physical stress in restrained plastics because of these high CTEs. Sudden temperature changes, even as small as 5ºC, can cause stress around fastenings or at the juncture with other materials. Freezing can be catastrophic if such a plastic is restrained in any way. When objects are moved to and from cold storage, the rate of cooling or heating should be such that the maximum temperature difference between parts of the objects (e.g. surface and core) is less than 6 to 10°C to prevent condensation (Padfield 2002). The importance of low rates of change of temperature becomes greater as the object thickness increases, because more time must be given for the object to equilibrate temperature across the greater thickness. Shashoua (2005) found no significant physical damage or condensation on thin walled objects (<1 cm / 0.4 in.) when cooled from ambient to freezer temperature at an uncontrolled rate. Condensation was detected on thick walled objects (> 1 cm / 0.4 in.) if temperature differences between the core and the periphery of the object exceeded 10°C. Controlled slow temperature changes must be maintained by gradual change of external temperature.

Recommendations

• Treat response of plastics to temperature as similar to that of natural organic materials.
  • Avoid hot lights (e.g. exhibition and photography floodlights).
  • Avoid proximity to any heat source (e.g. radiators, heating ducts).
  • Avoid any storage or display practice that heats objects.
• Reduce temperature to decrease the rate of degradation reactions, to slow the loss of volatile components, such as plasticizers, and to decrease the amount of creep and distortion.
  • Avoid flexing cold plastics, which become more brittle in decreased temperature.
  • Use rigid, padded, shaped supports for cold objects.
• Avoid fluctuating temperature to decrease damage from stress cracking of restrained plastics.

Damp, over 65% relative humidity

Damp is high RH but it is not wet, i.e. not covered by liquid water. Damp encourages mould growth if suitable nutrients, such as soluble protein, starch or sugar, are present. Table 1 in Agent of deterioration: incorrect relative humidity lists sensitivities of types of museum objects to mould. Synthetic polymers are not nutrients, so mould will not occur unless there is nutritious dirt like oils on the surface or the plastic contains nutrients like wood filler or plasticizers like diethyl sebacate, and there is sufficient moisture, in which case the plastic would have greater than medium sensitivity for mould growth. Most plastics have less than medium sensitivity.

Mould damage is usually restricted to unacceptable appearance such as staining, possibly accompanied by slight pitting of a thin surface layer by penetrating hyphae. Methods of dealing with mould can be found in Mould outbreak — an immediate response. Mould can be prevented by maintaining RH at less than 65%. There are no benefits to storage at greater than 65% RH, and there are few liabilities to less than 65% RH, so always maintain below 65% RH.

Relative humidity above 0%

Swelling

Swelling due to moisture uptake is quantified by the coefficient of moisture expansion (CME, also called coefficient of hydroscopic expansion and coefficient of swelling). This is the ratio of swelling to change in RH with the units of cm of swelling per cm of initial length per %RH (or microstrain/%RH, where [strain] = [change in length]/[initial length]). Sensitivity of plastics to moisture expansion (swelling) is shown in Table 9.

Plastics can only swell if they absorb moisture. A common measure of moisture absorption is the change in weight after immersion in water for 24 hours. This can also be a useful measure of susceptibility of plastics to damage after flooding. Water absorption of plastics is listed in Table 9.

Note that CME for plastics (less than 200) is very much smaller than for natural biological materials like ivory (205), oil paint (248) and wood (400 to 700). Plastics, except CF, are much less sensitive to swelling than natural biological materials.

Deliquescence of accretions

Some plastics, most notably highly vulcanized hard rubber (Ebonite, Vulcanite), CN, and CA, degrade by hydrolysis to produce ionic degradation products (sulfates, nitrates and acetates) that can react with atmospheric pollution or compounds in the plastic or surrounding materials to produce deliquescent accretions like ammonium sulfate, ammonium and calcium nitrate, and ammonium acetate (Figure 3). When the RH is greater than a specific value for each salt, deliquescent salts form droplets of salt solutions by dissolving in moisture absorbed from the air. Droplets of salt solutions on surfaces of plastics may damage other objects if they come into contact. Additional information on deliquescence is available under “RH above or below a specific critical value,” which can be found under Deterioration by incorrect relative humidity, and the collections most vulnerable in Agent of deterioration: incorrect relative humidity.

No plastics deliquesce, so this risk only occurs if there are deliquescent accretions on the plastics.

Relative humidity fluctuations

For a detailed discussion on the effects of RH fluctuations, see the section "RH fluctuations" under Deterioration by incorrect relative humidity, and the collections most vulnerable in Agent of deterioration: incorrect relative humidity.

Recommendations

• Prevent high RH that can cause mould. Consult Agent of deterioration: incorrect relative humidity for details.
• Reduce RH.
  • Low RH is better than high RH for all plastics because hydrolysis reaction rates are reduced.
  • Assume that dry plastics are brittle so handle gently, avoid flexing, and use rigid, padded, shaped supports.
  • Static electrical charge is enhanced at low RH so be wary of dirt pickup and disruptions of powdery and fibrous layers and surfaces.
• Avoid large RH fluctuations.
  • This prevents cracking of moisture responsive plastics.
  • RH fluctuation may cause surface crazing of UF.
  • Excursions to high RH might cause swelling and warping of CF.

Stage 1: remove obviously harmful plastics

• If a plastic is clearly causing damage to surrounding objects and storage materials (i.e. they are corroding, sticky, discoloured, sweating or tendered); or
• If the plastic itself smells acidic, sulfurous, or is oozing, sweating, blooming or crumbling:
  1. Remove the plastic to a cool, segregated, ventilated space (e.g. fume hood or shaded porch), to be dealt with later (Stage 3 and 4). Handle with care; it is probably fragile and may have a very acidic surface. Immobilize any crumbly foam or fractured brittle plastics so that they will not be unnecessarily damaged by handling and movement.
  2. Clean the affected area, line the area with a PET sheet, and replace contaminated storage materials.
  3. Arrange for the conservation treatment of any non-plastic object that has been damaged.
• If a plastic is odourless, is not causing visible damage to its surroundings, and looks intact, proceed to Stage 2.

Stage 2: check the undeteriorated plastics for acidic emissions

In many cases, deterioration may still be in an early, invisible stage. Check each plastic with indicators so its emissions can be detected and controlled before they cause damage. Objects should be exposed to the indicator for at least 24 hours to ensure sufficient time to detect emissions.

• Cresol red will detect strong acidic oxidizing acids such as those emitted by CN.
• Modified bromocresol green indicator papers (A-D strips) will detect weak organic acids and sulfurous, sulfuric and hydrochloric acids, as well as plastics contaminated by acid emissions from poor storage conditions. The test should be carried out in the dark because A-D strips fade readily to a pale greenish colour, which is often mistaken for a positive acid reading. Bromocresol green is rapidly bleached by oxidizing agents, so has limited use as a monitor for CN plastics (refer to use and indicator recipes below under Use and formulation for cresol red and modified bromocresol green indicators).
  • If the cresol red turns from yellow to pink, quarantine the plastic object in a cool, segregated and ventilated place (as above) to be dealt with later. Such a reading indicates that the object is already a severe hazard to the collection. Proceed to Stage 4.
  • If the A-D strip changes from blue to green then yellow, quarantine in a cool, well-ventilated area and proceed to Stage 4.
  • If no indicator changes colour, leave the plastic in storage and proceed to Stage 3.

Stage 3: identify potentially malignant plastics

At this stage, the actively harmful and visibly deteriorating pieces will have been removed from the collection and the remaining plastics will be (for the moment) quiescent. It will help in reducing the future workload to identify the potentially malignant plastics, since benign plastics will not need monitoring. Concentrate first on identifying objects containing CN, which is the most dangerous plastic in collections and needs special treatment. Fortunately, it is also one of the easiest to identify using spot tests. Reliable identification with spot tests takes practice so, if in doubt, skip Stage 3 and retest all plastics with indicators every six months, as in Stage 2.

By analysis or using spot tests (Table 3), determine which objects are malignant plastics.

• If the plastic is CN, leave in storage with a cresol red monitor (Figure 4). Check every six months and replace the monitor every two to three years.
• If the plastic is one of the remaining most malignant five, leave in storage and retest every six months with fresh indicators. (Unlike cresol red, A-D strips cannot be left as reference monitors.)

Stage 4: cope with corrosive plastics

Once the plastics in storage have been monitored, it is time to revisit the quarantined items. There are several storage options, depending on the nature, size and strength of the plastic object. Rubber or highly plasticized objects may become soft and sticky, so they should be kept well-supported and interleaved, as necessary, with non-stick materials such as SI. The correct precaution may be selected from the following options.

Sorbents

Some plastics emit volatile degradation products that may damage other objects nearby. It might be possible to control and lower the concentration of these emissions by methods used for collection facilities as described in Agent of deterioration: pollutants. For very emissive objects, or to control the immediate environment around the object, it may be necessary to isolate and contain the object in a sealed package. If so, the use of sorbents to remove emitted volatile degradation products will be necessary.

Sorbents can be used to remove emissions but they must be used with an understanding of their limitations or they can cause more damage than doing nothing. A major advantage in using sorbents is that the container of objects whose gases are being absorbed in a self-contained system can be returned to storage, provided there is regular monitoring.

i) Physisorbents (e.g. silica gel, activated charcoal, montmorillonite clays, zeolites)

Sorbents preferentially sorb specific compounds. To be effective, they must have selectivity for the pollutants, and have sufficient capacity to sorb large amounts of pollutants. For example, montmorillonite and zeolites are effective in controlling acetic and formic acid emissions from CA and UF. Physisorbents desorb as well as adsorb, so the adsorbed target pollutants may be displaced later by other gases, including moisture, or desorbed if the partial pressure of the pollutant drops and the pressure changes.

Physisorbents sorb water vapour. All gases surrounding a physisorbent compete for the reactive sites on the sorbent. In an atmosphere containing several gaseous components, the amount of each sorbed is approximately proportional to the concentration of the gas. At 50% and 25ºC, the concentration of water is about 10 g/m³ = 10,000 mg/m³. The odour threshold for acetic acid is 0.04 mg/m³. Thus, in a cabinet with a just perceptible odour of vinegar, the physisorbent will sorb predominantly water, not acetic acid. The corollary is that physisorbents will be most effective at sorbing pollutants in low RH conditions.

Method

A transparent box large enough for the object (or a tray supporting a double bag of a good vapour barrier such as PET) is lined with sorbent approximately one-third of the volume of the container. The object is placed on top, protected from direct contact with the sorbent by a cylinder of PP canvas or a shallow inner box. An indicator for the pollutant should be placed in the air space above the sorbent and close to the object, to enable monitoring that the sorbent is functioning and effective. The outer container is sealed.

ii) Chemisorbents (e.g. Ageless, calcium carbonate, or finely divided metal that removes contaminants by reacting with them to produce another substance)

Paper buffered with calcium carbonate has been used for many years to neutralize acids given off by CN and CA negatives by converting them into calcium nitrate and acetate. Unfortunately, its buffering action is not really effective at the low RH levels recommended for plastics, and the plastic object must not be in direct contact with the buffering agent because the reaction salts are deliquescent and absorb moisture from the atmosphere and liquefy.

Oxygen scavengers are suitable for PUR, rubbers, and plastics susceptible to photo and thermal oxidation. A simple and economic way of achieving oxygen-free conditions is to use an oxygen-absorbing material, such as Ageless, in envelopes or packages made from flexible, heat-sealed oxygen-barrier film (Figures 5 to 8). A disadvantage is the difficulty of knowing when to change the scavenger, because the Ageless Eye indicator is not very accurate. Oxygen scavengers are not useful for storing CN because the degrading plastic generates its own oxidizing agents and, more importantly, CN degradation occurs mainly through hydrolysis, not oxidation. Therefore, a better strategy for stopping hydrolysis is to reduce RH.

Rigid containers like bell-jars that are flushed and then filled with inert gases can be used as an alternative to flexible plastic packages. Hermetically sealed display cases can be constructed and filled with inert gas, but these are difficult to build so that all seals are perfectly gas tight. Contact CCI for further information on sealed display cases.

Scavengers with particulate metal, for example Corrosion Intercept, are suitable for UF, PUR and rubber. Finely divided copper metal incorporated into a PE sheet or zipper-sealed bag acts as both a barrier and a scavenger; acid from objects enclosed in the bag reacts with the copper particles as it permeates through the PE. There is a visible change from a metallic copper colour to a dingy green-black when the reservoir of copper particles is used up. This makes monitoring very simple. The disadvantage is that the object is not visible, so fragile items must be carefully supported inside the wrapping, and bagged items should preferably be handled by means of trays. It is not known whether the rate of acid removal is always fast enough to prevent a harmful concentration of corrosive gases in the bag. If the odour from the object appears to be very strong, additional buffering material can be included.

Corrosion Intercept comes in sheet form, so heat-sealed containment can be tailored to fit awkwardly shaped items. Vacuum extraction to remove air can enhance the protection, or other sorbents can be enclosed with the object. Corrosion Intercept reacts with volatile sulfur compounds as well as acids. In combination with Ageless, it is a very promising storage method for rubber and PUR because the copper particles make the PE into an effective light and moisture barrier. However, there should be no direct contact with oily exudates, such as found on some PUR and PVC objects. Oily exudates swell the PE, and in some cases, the interaction of copper and PVC plasticizers creates stains on the PVC object.

Stage 5: clean

Whether or not they are malignant, plastics from storage rooms are often dirty and may have sticky exudates that eventually harden and discolour. There is controversy about whether blooms and dirt should be removed. Given the state of knowledge at present, it is best not to clean unless the object is to go on exhibition and there is no substitute or replica that can be used instead.

Research on the cleaning of plastics by using dry mechanical methods with dry materials like cloths, sponges, brushes and cotton buds, and by using wet mechanical methods with the same cleaning materials moistened (not sopping wet) with aqueous cleaning agents containing dissolved agents like surfactants and chelating agents, and with organic solvents was recently reported by Balcar et al. (2012). Detailed information is given on the effects of these cleaning materials and agents for CA, HDPE, high impact polystyrene (HIPS), PMMA, and plasticized PVC.

The predominant parameters observed and evaluated after cleaning were scratching, change of gloss and cleanliness.

The main points revealed are that:

1. Rubbing with dry cleaning materials tend to cause at least 10 times more scratching than the same cleaning materials moistened by cleaning agents (so long as the agents themselves do not cause damage). This was attributed to increased lubrication by the cleaning agents, but also could be due to softening of the cleaning materials by the cleaning agents.
2. Cotton buds and microfibre cloths (polyester plus nylon) tend to be much more suitable than brushes and sponges when used with aqueous and organic solvent cleaning agents.
3. High polarity solvents like acetone are damaging and unacceptable for cleaning any plastic.
4. Least damaging but still effective organic solvents are those of low polarity such as white spirit, ethanol, isopropanol and xylene.
5. Aqueous solution of surfactants and chelating agents are more effective for oily soils than organic solvents, but organic solvents are more effective than surfactants and chelating agents for waxy soils.

The best approach is to clean objects under dry conditions by careful brushing or vacuum cleaning. Sable hair brush, cotton buds, lens cleaning tissue made from pure cellulose or unimpregnated microfibre cloths are quite effective for wiping off surface dirt and bloom. Microfibre cloth of the kind specifically made to absorb oil is excellent for removing soaps and oily plasticizers. Do not use the same wipe on more than one plastic or more than one object for fear of cross-contamination with incompatible materials. Dispose of used lens cleaning tissue and buds and launder microfibre cloths very well. If simple techniques do not work, custodians are strongly advised to seek expert help from conservators.

In general, the recommended sequence for cleaning is:

1. Remove dust by blowing clean air over the surface or vacuuming, assisted by sable hair brush.
2. Remove loose particles with cloths (microfibre cloth is best) or cotton buds with a minimum number of gentle wipes applied with minimum force.
3. Wipe with cloths or cotton buds moistened (not sopping wet) with distilled water.
4. Wipe with cloths or cotton buds moistened with aqueous surfactant or chelating agent solutions.
5. Wipe with cloths or cotton buds moistened with ethanol, isopropanol or white spirit.

Cleaning agents that were tested and are recommended are:

• 1% (w/w) non-ionic detergent (e.g. Dehypon LS45; Judith Hofenk de Graaff detergent concentrate: 50 g sodium dodecylbenzenesulphonate, 50 g triammonium citrate, 5 g carboxymethylcellulose in 1000 ml distilled water)
• 1% (w/w) anionic detergent (e.g. Orvus WA Paste)
• 2.5% (w/w) triammonium citrate

Never use oils, bleaches, wax, or vinyl and rubber erasers; these materials are no longer recommended because they can accelerate deterioration.