Archival Products NEWS Volume 15, No. 3
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VOLUME 15 NO.3 2008 Archival Products NEWS Fire and Ice: A Soot Removal Technique Using Dry Ice Blasting By Randy Silverman To say that for destruction ice Among U.S. Is also great And would suffice commercial —Robert Frost 1 disaster firms, the current WELDER’S SPARK TOUCHED off the attic fire in the Sevier standard for ACounty Recorders Office in removing soot’s Richfield, Utah on May 2, 2006, igniting residue from a blaze that ripped through the crawl space books is to consuming the building’s paper-backed insu- Front of the Sevier County Recorders Office (Richfield, Utah, wipe down the lation. The fuel readily spent, the fire burned USA) on May 9, 2006, one week after the fire. Photo credit: itself out 15 minutes later, sparing the struc- Randy Silverman covers with a ture but coating everything below the rafters “chemical,” or with fine, powdery soot. The Problem This carbonaceous residue filtered down Among U.S. commercial disaster firms, the natural rubber through the ceiling tiles settling on everything current standard for removing soot’s residue sponge, and in the offices below including the historic from books is to wipe down the covers with a then “ozonate” 2 courthouse record books stored horizontally “chemical,” or natural rubber sponge, and then the books to on metal rolling shelves within the vault. “ozonate” the books to eliminate the residual About 300 nineteenth and twentieth century smoky odor. This approach leaves much to be eliminate the full-leather spring-back stationers’ bindings, desired. While the sponge traps much of the residual smoky many covered in protective white canvas jackets, fine, carbon-laden particulate in its tan, rubbery odor. were untouched by the fire but impregnated surface, it also quickly fills with residue. Recovery with a layer of soot and reeked of smoke. workers must constantly rotate their sponges to F IRE AND I CE – continued expose unused areas to the grime and discard oily foundation of partially consumed combus- Soot’s fine spent sponges as they are not readily cleaned. tion byproducts. The carbon within these tar powder coats The wiping process forces some of the fine, droplets is so fine that it is readily dispersed by every exposed dark soot particles back into the interstices the “pressure and buoyancy created by the heat of the material being cleaned, especially when of the fire” and aerodynamic conditions such as surface, pene- that surface is as porous as the open weave “stack effect, wind pressures, the building geom- trating even the of canvas book jackets. etry and its barriers (such as walls and floors), tiniest crevices Trapped soot becomes more intractable and ventilation practices.”5 Soot’s fine powder and crannies, with time as the polymerized and dehydroge- coats every exposed surface, penetrating even nized byproducts of the fire chemically bond the tiniest crevices and crannies, anchoring the anchoring the to their surroundings.3 The friction of wiping carbon with oily tars where it lands. Removal carbon with also causes some portion of the sponge’s soft attempts by wiping, even with an absorbent, oily tars where rubber to transfer to the book’s surface, trading fleshy material such as a natural rubber sponge, one unstable residue for another.4 Finally, smears whatever soot does not bond to the it lands. exposing “cleaned” books to ozone to reduce sponge, compressing and embedding the the residual smoke odor causes further degra- diminutive, greasy specks further into the sur- dation. A strong oxidizer, ozone aggressively face and making them more difficult to remove. breaks down paper, cloth, leather and adhesives As soot ages, it chemically cross-links to the while it decomposes the organic components material it is in contact with, making immediate of the smoke—a highly undesirable tradeoff for cleanup the optimal course of choice. books of historic significance mandated by law Vacuuming with a HEPA filter in tandem to be maintained in perpetuity. with manual wiping can help, but alone it is In short, soot is an extremely tenacious mate- actually less effective than wiping with an rial to remove. Unlike dust, it is a solid/liquid absorbent material. In the 1997 Saskatchewan residue composed of carbon suspended in an Museum fire, Spafford-Ricci and Graham report the soot removal protocol used for book cleaning included an initial vacuuming of the book’s binding, followed by a separate vacuuming of the text block. Care was taken not to touch the surface of the books with the vacuum’s nozzle as this contact would push soot into the woven fabric of the bookbindings. After vacuuming, the second phase of this cleanup included mechanically wiping the book’s surface with rubber sponges or Webril® Wipes (a nonwoven 100% cotton pad used in the printing industry for non-abrasive cleaning of printing plates).6 Eliminating residual smoke odor from objects following soot removal is the next problem. In addition to spraying scents to mask the odor, three approaches predominate within the fire recovery industry: chemical deodorizing, thermal deodorizing and ozone treatment. Unfortunately, all have serious An interior office with evidence of smoke damage on the drawbacks when dealing with cultural her- walls. Photo credit: Randy Silverman itage material. 2 F IRE AND I CE – continued Chemical deodorizing eliminates odors through a chemical reaction occurring when the chemical fumes of the product come into contact with smoke residue. Record book storage vault with metal roller shelving; a temporary positive air ventilation system is set up to help with soot extraction. Photo credit: Randy Silverman Chemical deodorizing eliminates odors printing, paint, food, sewage and waste treat- through a chemical reaction occurring when ment, application of this technology to reduce the chemical fumes of the product come into smoke odors from heritage materials has not contact with smoke residue. These deodorizers yet, to my knowledge, been attempted. come in a wide range of extremely pungent Finally, ozone is commonly used to treat fragrances designed to “purify” air spaces rang- smoke odors in affected household and office ing in size from 1,000-20,000 cubic feet. The objects. This treatment includes creating long-term effects on cultural property of these copious amounts of O3 with an electric proprietary formulations have not been analyzed. ozone generator and sequestering the smoke- More broadly, however, harmful effects from damaged material in a confined space with gaseous pollutants—particularly sulphur the gas. Unfortunately, while O3 eliminates dioxide (SO2), oxides of nitrogen (NOX) and smoke odors, in high concentrations O3 9 ozone (O3)—have been well documented with is both harmful to human health and an paper, leather, textiles, dyes, pigments, inks, aggressive oxidizer known to harm a wide adhesives and photographic film.7 Introduction range of cultural heritage materials, as noted of gaseous chemicals for deodorizing purposes above.10 Hence, despite its common use for is not recommended until their long-term less significant objects, O3 should be avoided. effects can be tested. At present, the only safe approach to Thermal oxidation deodorizing is a second remove smoke odors from cultural material is approach used to eliminate volatile organic to isolate objects in a room with an operating compounds (VOCs) from the ambient air. air purification system containing activated Essentially, the system is an afterburner that carbon, zeolite and/or potassium perman- draws workplace air through a combustion ganate filtration. Continuously re-circulating chamber where VOCs are incinerated.8 filtered air past the objects will gradually Mostly used in industrial settings to deal reduce smoke’s lingering odor if the material with gaseous byproducts from petrochemicals, can be well exposed to the air flow. An inex- 3 F IRE AND I CE – continued pensive alternative is to place small amounts of including dispatching paint from decorative In the aftermath material in relatively airtight enclosures (such metalwork; cleaning dirt from brick, granite, of the Sevier as a large, sealed plastic garbage bag) in close marble, onyx or other stone materials; stripping County Recorders proximity to large quantities of exposed baking built-up wet or dry ink from printing presses; Office fire, a soda. Either approach may take two to three removing fused dust from electrical turbines, months to work with the key being to expose generator windings and transformers; and non-damaging the impacted material to the absorbent media. remediating mold from building interiors.13 alternative to the Michael Trinkley reports, “Records which sur- The system is portable and can be powered current cleaning vived the 1906 San Francisco fire and are today by an electric generator, adding significantly in the National Archives still smell strongly of to its merits since the County Recorder would options seemed smoke—over 90 years later,”11 a condition not permit the damaged books to leave the desirable. possibly exacerbated by the lack of air exchange. Recorders Office. The process works by shaving solid blocks Sevier County of dry ice (frozen CO2) into granules ranging in In the aftermath of the Sevier County Recorders size from the diameter of sugar to the shape of Office fire, a non-damaging alternative to the rice, depending upon the application. These current cleaning options seemed desirable. granules are propelled in a compressed air Invited by the responsible commercial recovery stream of 30-300 PSI against the surface company to serve as a consultant,12 I suggested to be cleaned. Dry ice blasting is considered dry ice blasting as an interesting possibility. Dry completely non-abrasive when used on surfaces ice blasting has proven its utility in a variety of harder than frozen CO2, but can be used to industrial applications over the past decade “antique” wooden siding by differentially abrading the softer, pithy wood and leaving the grain.