Sometime in early 1996 a compressed-air spray system for use with the Bookkeeper deacidification fluid was installed at the Northeast Document Conservation Center. We are presently using both Bookkeeper and Wei T'o and choose between the two materials according to the kind of paper and the specific media being treated. Highly calendered and sized papers are usually treated with Wei T'o because of Bookkeeper's tendency to leave magnesium oxide deposits on the surface of nonporous papers. Colors that may shift with Wei T'o have been successfully treated with Bookkeeper. Bookkeeper's ease of use has caused many conservators on staff to prefer it to Wei T'o although it is more expensive. On the other hand, where a slight darkening of paper is acceptable and cost is a determining factor in deciding whether or not to treat, Wei T'o is a reasonable alternative for us. The objects treated with both agents include documents, maps and bound volumes.
The equipment described below and listed at the end of this article serves two departments at NEDCC. A vacuum generator, air brushes, and the Bookkeeper spray system in the lab's fume hood room, and a dust removal system in photoduplication, are all powered from the same source.
Some of the items in this system are specific to our particular needs and may have greater capacities than required for a less ambitious installation. For instance, the refrigerated air dryer might be eliminated if the air tank were bled after each use. In the case of a compressor, a 1-1/2 horsepower oil-less compressor running on a 110-volt circuit with its own air storage tanks would likely suffice for low to moderate use.
The compressed air system at NEDCC consists of a 2-horsepower oil-less piston compressor, a 60-gallon air storage tank, a refrigerated air dryer, two air filters, a 2-gallon single regulator paint tank with agitator, and a high pressure spray gun. The system is connected by 1/2" copper pipe except where flexible hose is necessary or more practical. Most components of the system can be purchased from local suppliers and should be considered generic. We tried several spray guns and settled on the model used in the Wei T'o soft spray system (Gunjet spray gun AA23H-SS from Spraying Systems Co. in Wheaton, IL). We are using a smaller spray tip to reduce waste of the Bookkeeper suspension (Tee-Jet 730023-SS flat-spray pattern, also from Spraying Systems). The nylon fluid hose from paint tank to spray gun has a swivel connector at the gun which has proved very convenient. The air filters are an integral filter and dryer and a submicronic compressed air filter.
Initially the system was very loud even though the compressor was located in a closed room. Installation of a flexible line from the compressor to the air storage tank and addition of mounts on the compressor and an insulating blanket on the air storage tank have reduced the noise to an acceptable level. During spraying, the compressor and the pipe attached to it become hot quickly. The compressor should be installed where nothing flammable will be stored near it and where people will not need to pass by it, reach over it, etc. Ideally the compressor, air dryer, and storage tank should be isolated in a noisy room. The air dryer is turned on with the first use and shut off at closing time each day.
The paint tank lid is heavy and the agitator extends deep into the paint tank. It is necessary to lift the lid high enough for the agitator to clear the paint tank when filling the tank. We have installed a heavy duty hook on the fume hood on which to hang the lid while filling the tank (the lid comes with a large stainless steel hook). Because Bookkeeper is also very heavy (about 20 pounds per gallon), a dolly for the paint tank is very practical. The optional tank liner makes getting a good seal difficult and we no longer use it. There are quick disconnects on either end of the fluid hose. We also installed a shutoff valve between the tank and the fluid hose to prevent the fluid from entering the hose before it is stirred.
Cost of installation not including labor was about $3000. Approximate costs for the major components were: compressor, $1,100; air storage tank, $300; refrigerated air dryer, $625; air filters, $200; paint tank, $650; and spray gun with tip assembly, $150.
Since Bookkeeper is a suspension of magnesium oxide particles in an inert carrier, the magnesium oxide settles out over time. Before adding Bookkeeper to the paint tank, it should be shaken until all the white sediment on the bottom of the plastic jug is in solution. The paint tank seals with four wing nuts, which should be tightened laterally to ensure a good seal. With the valve to the hose closed, we pressurize the tank and turn on the agitator. The agitator uses a lot of air, but it only needs to run at full speed for a few minutes before spraying begins in order to disperse the magnesium oxide particles in the carrier. Then it can be turned off and run for two minutes every 20-30 minutes.
We are presently spraying with a pressure of 28-30 p.s.i. Bindings and page edges are protected from overspraying by masking with paper just as they would be when spraying Wei T'o. To get proper coverage, we spray 10-12 inches from the object at a speed of about one foot every three seconds. The centers of the sprayed areas on large objects requiring more than one pass should be about eight inches apart. We spray both sides of the object whenever possible. With this method, surface pHs tend to be between 8.0 and 8.5 when test spraying brown wrapping paper with an initial surface pH of 5.0 to 5.5. Coverage in the gutter area of books appears to be good using this method as evidenced by volumes marked with pH pens before spraying. A white powder on the surface of the object may result from spraying too slowly; the desired higher pH may not be achieved if the object is sprayed too quickly. Under-sprayed objects yield dots of color on pH strips indicating both alkalinity and acidity in one small area.
After spraying, the liquid in the hose is drained back into the tank. We turn off the air supply to the tank, open the pressure-release valve, hold the gun high to extend the hose, and pull the trigger to allow air into the gun, being sure to drain the hose completely. Then we close the valve between the tank and hose to allow proper mixing the next time the unit is used. When not in use, the valve to the agitator is also closed to prevent possible evaporation of the carrier.
We spray only in an operating fume hood and wear latex gloves. In the material safety data sheet (MSDS), the manufacturer describes the materials as nonhazardous. However, the material has an odor and coats paper with a fine white powder after repeated spraying. The tiny magnesium oxide particles are obviously airborne during spraying and could easily be inhaled if the material is not used with proper ventilation. The MSDS states that no respiratory protection should be required at ordinary room temperature as long as prolonged breathing of the vapors is avoided and the material is used "with sufficient local exhaust ventilation to maintain airborne concentrations at recognized health and safety levels." The MSDS also states that "prolonged contact may cause mild skin irritation."
So far, the fluid hose has not required cleaning. The screen and spray tip can be cleaned very quickly with a soft-bristled toothbrush. After we abandoned the tank liner, we found metal shavings in the tank which were successfully removed with a magnet. Presumably these were the result of the manufacturing process since no shavings have been detected since. Small hard black particles (which seem to be rubber and are probably from the gaskets in the gun) have been trapped in the strainer of the spray tip assembly. This will be something to watch. It's possible that the magnesium oxide particles are degrading the gaskets, in which case frequent replacement of the rubber parts may be necessary. Use of a screen is clearly important. Finally, the gun does not shut off immediately when the trigger is released, so that a small amount of liquid squirts from the gun after spraying. We have not been able to solve this problem despite consultation with suppliers. We check the air filtering system every six months but have not needed to change or drain the filters to date.
The prices quoted are those listed by the suppliers named as of early 1996.