3 3. GENERAL GUIDELINES FOR REMOVAL OF SOOT FROM MUSEUM OBJECTS

The RSM fire was particularly sooty, and conservators spent months studying the peculiarities of handling and cleaning soot-damaged objects and carrying out research into postfire cleaning reports cited in the conservation literature. They were later able to apply this knowledge to the removal of soot from cultural objects involved in other fires. These experiences have refined general salvage and cleaning procedures that are extremely effective in mitigating the damage caused by soot deposition, particularly if the soot layer to be treated has not been disturbed. Recommendations for soot-removal treatments are outlined below, and recommendations for the postdisaster salvage, handling, and temporary storage of sooty objects following a fire are presented in another article in this issue (Spafford-Ricci and Graham 2000).

3.1 3.1 CHARACTERISTICS OF SOOT

Soot represents the liquid and solid fragments of pyrolysis. It may be composed of hundreds of different compounds that will vary depending on the materials that are pyrolyzed in a fire. Regardless of the nature of these compounds, all soot is composed of an oily-tarry matrix combined with carbon and, as such, has predictable characteristics. Soot particles in smoke may be as small as 1 μm in diameter and therefore are often not distinguishable under a compound microscope. The substance that is visible to the naked eye is a soot web formed when the oily materials surrounding the carbon black are attracted to one another and form agglomerations. During a fire, soot agglomerations are segregated in the air due to flotation effects, the larger agglomerations dropping out of the air closest to the fire and the finer agglomerations farther from the fire. Soot particles will penetrate the finest crevices of a surface and remain physically trapped, their attachment enhanced by electrostatic attraction. In solution, the carbon can reach an atomic scale of fineness that will then be redeposited in this size when a cleaning solvent evaporates. Soot will embed into porous and compromised surfaces such as those melted by the heat of a fire (Williams 1990).

These characteristics of soot account for the phenomena that the RSM conservators noticed while in the preliminary stages of fire recovery. After completion of cleaning tests on a white pelican, it was noted that the threads used to separate different cleaning tests had left thin gray lines where they had pressed the soot into the feathers, and these lines could not be removed. Also, if an inappropriate wet-cleaning agent was used on a varnished acrylic diorama painting, a gray layer remained and also could not be removed.

3.2 3.2 REMOVAL OF SOOT USING A PROGRESSIVE CLEANING TECHNIQUE

The impact of the qualities of soot on cleaning techniques is illustrated by the ease with which soot, more so than almost any other particulate matter, can become ingrained in the surface of an artifact. For effective removal of soot, there must be mechanical and solvent action to clean but without breaking up delicate agglomerations of soot into yet finer particles. Cleaning techniques should be designed to avoid embedding particles into a surface either mechanically or through the use of organic solvents that will extract oily components that can be absorbed into an artifact surface along with the particles of carbon black.

The acidic nature of soot adds a degree of urgency to its removal, as does the observed phenomenon that soot becomes more strongly attached to surfaces over time. Testing at the RSM showed that some objects were more difficult to clean six weeks after a fire than they were after only one week. Conservators have theorized that this effect may be due to cross-linking, but it may also be related to the physical compacting of a soot layer over time. Generally speaking, removal of soot during recovery after the RSM fire became even more difficult if the soot layer had been compacted through excessive handling or movement, if the object had been subjected to high humidity conditions, or if an unsuccessful cleaning attempt had been made.

At the RSM, response of soot to cleaning treatments varied tremendously depending on the type of material surface that was being cleaned. In addition, in different areas of the museum, and indeed on different areas of the same object, differential heating called for the use of different cleaning regimes. Nevertheless, the conservators found that the predictable characteristics of soot make it imperative that certain general guidelines be followed during the removal of soot from objects following a fire.

RSM conservators found that a strict progressive cleaning procedure that begins with vacuuming and moves to dry-surface-cleaning, then to wet-cleaning, will guarantee the highest degree of soot removal for most objects. This success is particularly true of soot loosely to moderately bound; for cleaning of more tightly bound layers of soot, the procedure is less critical but still applicable.

3.2.1 3.2.1 Step 1: Vacuuming

Removal of soot by vacuum should be the first step in a soot removal treatment. In a postdisaster situation, the highest degree of soot removal is possible only if an artifact is vacuumed before it is touched and certainly before it is packed and moved. Vacuuming should be carried out with direct use of the vacuum nozzle and, wherever possible, without the use of a screen or brush, which may embed soot in the surface. The vacuum nozzle should not touch the surface of the object. For firm objects, it is possible to hold a crevice tool above the surface of the object, propped on a finger, which is passed over an already cleaned area. A circular vacuum cleaner attachment (a nozzle can be made by cutting back the hairs on a brush attachment) can also be used directly on a surface, which is then pulled upward to repeat the process in an adjacent area. To avoid dispersing a soot layer, it is essential that an object be vacuumed “as it is found.” For example, a textile that is folded should not be unfolded. Surfaces should be vacuumed first, and then cleaning can proceed to inner areas and crevices. To provide for the most controlled movements, vacuuming is best done with assistance; one person may hold a painting while another applies the vacuum. Portable canister vacuums such as shop vacuums or small canister vacuums should have a long length of hose complete with a crevice tool, a large circular nozzle, and mini-attachments as required. At the RSM, a vacuum with a HEPA filter was used to clean natural history specimens, but ordinary canister vacuums worked admirably for general soot removal from other objects. No vacuums burned out or encountered mechanical problems. The vacuum bags had to be changed frequently, and vacuum attachments required repeated washing to decrease soot transfer.

3.2.2 3.2.2 Step 2: Dry-Surface-Cleaning

Mechanical and dry-surface-cleaning materials are indispensable in the removal of minute soot particles. Depending on the surface to be cleaned, appropriate materials might include the use of erasers: eraser powder such as Skum-X, block erasers, particularly art gum and vinyl erasers, and mechanical erasers (Selick 1996). Elizabeth Moffatt (1992) points out that many of the dry-surface-cleaning materials that are particularly suited to soot removal are composed of vulcanized rubber: Skum-X eraser powder, Groom/Stick Molecular Trap, and soot sponges. Where bulk cleaning procedures are appropriate, an object can be cleaned in a tray of eraser powder. Fine glass beads (e.g., B.T. 13) can be used to lift and hold soot from a surface. Groom/Stick Molecular Trap has a particular ability to pick up and hold soot and is useful for surfaces that are porous or textured or have tiny recesses. Groom/Stick can be used by hand or applied on the end of an applicator stick or other tool. Soot sponges are very useful for cleaning many surfaces and can be used full size or cut into smaller blocks. Cotton batting and soft wipes such as the brand Webril Wipe or Dust Bunny can be used in a gentle lifting motion; broad rubbing motions should be avoided unless a surface is not porous or textured. Dry-surface-cleaning materials are also useful when testing for the presence of a light soot layer on an object. A small block of soot sponge or Groom/Stick rubbed onto a lightly sooty surface will pick up some soot, where a swab dampened with a cleaning solvent will not.

3.2.3 3.2.3 Step 3: Wet-Cleaning

The particular characteristics of soot and the way it bonds to surfaces indicate that vacuuming and other dry-surface-cleaning methods are the first lines of defense in soot removal. Wet-cleaning can be less successful, particularly if the unique characteristics of soot are not taken into consideration. Because of the differing sensitivity of surfaces to wet-cleaning methods, it is difficult to generalize about these cleaning methods for the removal of soot. Nevertheless, it should be noted that the water-repelling nature of soot points to the preference for a surfactant or detergent in water to remove soot, where it is appropriate. The extraction of colored materials with the use of organic solvents points to the need for caution with the use of organic solvents, particularly if not preceded by other cleaning methods. During wet-cleaning of a tightly bound soot layer, the complex components of soot should not be solubilized unless there is a mechanism to “lift” these components away from the surface immediately.

Generally speaking, wet-cleaning to remove soot should be carried out only when appropriate, after preliminary vacuuming and dry-surface-cleaning methods. Soot removal at the RSM was most often accomplished with the use of simple aqueous mixtures incorporating a neutral detergent such as Orvus or a surfactant such as Aerosol OT. In this postdisaster situation, these solutions were applied on swabs or pads of Webril and for bulk cleaning on pieces of sponge, in pails, or spray-applied. Other common cleaning solutions such as 1–4% ammoniated water, 1–3% sodium perborate and 1–2% diammonium citrate were also useful for removal of soot layers. Pure solvents such as trichloroethylene, Stoddard solvent, and ethanol alone or with the addition of a small amount of water (sometimes with a surfactant) were used at the RSM on surfaces where aqueous cleaning was not appropriate.

The degree of change to the surface of an object will affect removal of soot. A surface that has softened and imbibed soot adds a complexity to the removal of soot intimately tied with the surface. In this case, it has been noted that organic solvent gels are often required to loosen soot and can be used alternately with aqueous solutions to retrieve released soot (Henry 1995). At the RSM, organic solvents were required to clean feathers and furs. Varnished paintings and contemporary paintings are other examples of surfaces that may require organic solvent solutions if the surface is compromised by high temperatures; on the other hand, canvas will respond better to vacuuming and dry techniques before the use of any solvents.