September 1997 Volume 19 Number 3

Technical Exchange

Testing for Soluble Epsomite Salts in the Presence of Gypsum

Conventional microchemical spot tests are unable to confirm the presence of epsomite (magnesium sulfate, Epsom salts) contaminating a calcium sulfate plaster substrate. The slight solubility of calcium sulfate in water yields a positive test for sulfate with a barium chloride test masking the presence of a soluble sulfate salt. Magnesium and calcium anions are so similar chemically that they are very difficult to distinguish.

You can, however, purchase some very sophisticated chemistry in the form of a water hardness test kit, that can differentiate between calcium and magnesium in solution. The LaMotte Hardness Test Kit contains a mini-titration lab that allows total water hardness (calcium and magnesium) and calcium hardness to be determined. The difference between the two values gives the magnesium concentration.

The trick to using this test is to convert your plaster sample into a solution. A very small sample of plaster is removed mechanically and dissolved into 40 ml distilled or deionized water. Crush the small plaster sample and stir it around--it should be totally dissolved.

Fill the titration tube to the line (12.9 ml) with the solution. Follow the directions and test for calcium hardness and write down the result. (If you have to add more than two full tubes of the titrating solution from the graduated syringe, you have used too large a plaster sample. Dump it all and start again using a smaller plaster sample.)

Rinse out the tube and fill again with the sample solution. Perform the total hardness test according to the directions. Write down that number. Rinse the tube again and re-test the calcium hardness test and, again, note the results.

Compare the two calcium hardness tests. If they are the same, the sample was properly dissolved. If they differ, the calcium sulfate had not equilibrated with the solution. To a rough approximation, you can probably get away with averaging the two values. (You can also try repeating the test, but this time filter the solution before performing any of the tests.)

Subtract the calcium hardness value from the total hardness value and you have the magnesium concentration. If the magnesium concentration is greater than the uncertainty in the measurements, you probably have epsomite present in the plaster sample. Be sure to run a blank total hardness test on your distilled water to make sure it is not contaminated.

Hardness Test Kit
(Model PHT-CM-DR-LT)
LaMotte Company, P.O. Box 329
Chestertown, MD 21620
(800) 344-3100
or any good chemistry supply company.

Bogen Rail Lighting System

After years of tripping over light stands and extension cords we have recently installed an overhead rail lighting system made by Bogen for photography. This purchase has unexpectedly been one of the single most important improvements ever made to our conservation studio. The system we've installed consists simply of two fixed seven foot rails mounted to the ceiling which support a sliding nineteen foot main rail. The main rail in turn supports two sliding telescoping variable length posts that hold multiple light sources. The lights are pulled down from the ceiling and can be positioned effortlessly at any point within the seven by thirty odd foot area. Photography has actually become fun.

Among the benefits of the new system are its ease in handling (it routinely saves us hours a week), the increased safety (no more extension cords, light stands, or clumsy, floor-mounted objects), and the ability to light other areas of the studio using the same track. The entire system described above cost a little over $1,100.00 and was relatively easy to install ourselves.

There is an incredible variety of rail fixtures, brackets, adapters, and accessories which can be used to customize this system to the needs of any studio setup. It can be integrated into pre-existing systems, it functions well around many cumbersome ceiling fixtures, and it accepts a variety of different lighting units. It is also a system that need not be used solely for photography. Because of this favorable experience, we are considering installing a similar unit for inpainting stations and general task lighting, adding much versatility to our studio lighting.

"The Bogen Pro Rail Studio System" catalogue can be found at most large professional photography retailers.

PVA (OH) Sponges

Peregrine Brushes and Tools has been offering these truly remarkable sponges, and I couldn't possibly add to what Monica says about them:

"These are amazing! Super absorbent, dripless. Great for cleaning paintings, murals, smoothing gesso fills, cleaning water gilding, humidifying. This sponge has a very dense, compact structure. I'm told these sponges are a cross-linked PVA alcohol and not acetate (in Europe, PVAC is acetate); this is quite solvent resistant. The sponge must be soaked in water first; can be used with xylenes and ketones with intermittent rinsing in H₂O to which a small amount of something other than Triton X100 is added. Can be carved into specific shapes (such as a cylinder and rolled over surfaces)."

For more information, contact Monica Jaworski, Peregrine Brushes & Tools by phone or fax at (619) 231-4019.

Construction and Use of a Thin Profile Suction Platen

The use of a suction platen can facilitate many of the procedures used for the treatment of paintings, paper and other flat, porous objects. After having tried several home-made and manufactured units, I found that even the most technically sophisticated ones fell short of some of the requirements desired in such a device. I decided to build a model of my own to improve on these designs. Most notably the following criteria were crucial to me. The device should be thin so that it can be easily slipped between the reverse of a canvas and the stretcher bar of a painting. While remaining thin, there should be sufficient and even suction across the entire surface (all models I have seen fail miserably at this). It should be light weight and easy to position both working flat on a table and vertically on an easel. It should be made primarily of inert and solvent resistant materials. It should be easy to service, in particular easy to take apart and clean. Lastly, it should be affordable to make and easy to use. A considerable amount of time was spent experimenting with different materials and constructions resulting in a design that fulfills these requirements. Most of this time was spent devising a baffle for the center of the unit to distribute the suction evenly across the surface of the platen.

The platen consists of a top perforated plate, a baffle to even out air flow, and a base with a fitting to allow vacuum hose attachment and an attachment for clamping.

Perforated plate: 8" x 8" 22 gauge stainless steel (or aluminum), 1/16" diameter holes, staggered with 3/32"centers. This is the most open space with 1/16" holes available.

Baffle: three pieces of Poly-mesh with a piece of G-10 interlayed as an air flow diverter. The order from the top down: 4" x 4" Poly-mesh; 5" x 5" G-10; 7" x7" Poly-mesh; 4-1/2" x 4-1/2" Poly-mesh. The piece of G-10 should have the edges sanded smooth and the corners rounded. It should have 12 holes punched in it, with a hole punch: four located 1/2" in at each corner; eight located two on each side, 7/8" in from the edge and 1-3/4" in from the side. These elements are stacked up, centered, and held together with monofilament.

Base: The base consists of an 8" x 8" piece of G-10 reinforced with a piece of 1/8" Plexiglas, adhered with caulk or epoxy to the bottom surface at the center. Smooth and bevel the edges of the Plexiglas. A hole 2" diameter is cut (in each piece prior to gluing) to allow for the gluing in of the PVC pipe fitting. After final assembly a 2" C-clamp is attached to the PVC pipe fitting by means of a screw through the clamp into the fitting and a hose clamp for further reinforcement. The C-clamp is used to attach the device in position.

Assemble by taping tightly together the perforated plate and the base on three sides with strips of tape (aluminum backed duct tape or your tape of choice), the tape should wrap around the top surface 1/8". Insert the baffle and position it at center. Tape the last side tightly. DO NOT WORRY IF THERE IS A BIT OF A WARP TO THE PLENUM. This slight warp actually reduces the edge effect that some platens tend to cause.

The device can be easily clamped to an easel or a table edge. For easy positioning, clamp it to a camera tripod. To use horizontally, the platen can be attached to the tripod and set near the edge of a table with the painting hanging over the table and the free end of the painting propped up with an easel or saw horse. Steven Prins made a table out of Gator foam and saw horses with a 3" x 8" hole cut in it. The unit can be cranked up through the hole with the painting positioned over it.

For the suction source a 1x1 Shopvac^� works well (look for the new yellow kind; it has a wall mounting unit and 1-1/2 horsepower vs. the 1 horsepower of the older red models). Shopvac^� also makes a 7 ft long 1-1/4" diameter extension hose which comes with an attachment allowing it to be used with either a 1-1/4" or 2-1/2" hose. To muffle the sound without overheating it. I put my Shopvac^� in a nearby closet. The doorknob was removed so the cord and hose can come into the studio through the closet door. The amount of suction can be regulated either with a rheostat or by simply allowing air through a hole in the tube. Part of the platen can be blocked off to increase the pressure if necessary.

Two words of caution: Using even moderate amounts of solvents with a vacuum that is not spark-proof can create an explosion! Also when using adhesives, be very careful not to adhere your painting to the platen. To avoid this problem, place a piece of silicone coated Mylar or paper on the platen, turn the suction on to hold it in place, and then perforate the Mylar or paper with a fabric marking wheel. (Those things that look like a miniature pizza cutter with a jagged little wheel).

The platen has proven to be one of my most used and best loved tools. It is useful for consolidation, setting down lifting paint (especially when used in conjunction with a hot air tool), reducing planar distortions with moisture and heat (such as stretcher bar marks and corner draws), and holding tears in plane when re-aligning and mending them. I have even been successful in reducing fairly severe cupping by relaxing the paint with heat and introducing adhesives into the cracks.

Although this model does not have a built in heat source, by using heat lamps, hair dryers, or hot air tools, this has not been a problem. In fact, unlike platens which warm from the reverse, the heat is introduced where it is needed, and sucked through the painting rather than away from it. if the heat is introduced gradually with a heat lamp, the plenum stays warm and regulates the heat because it is made of metal. If you have any questions, comments, or suggestions or would like a platen already assembled, ($200), please do not hesitate to write or call.

MATERIALS:

Perforated plate: 22 gauge stainless steel (or aluminum), 1/16" diameter holes, staggered with 3/32" centers. Sometimes small pieces can be found at local machine shops or sheet metal fabricators. Large sheets, 3' x 10' are available from: Harrington & King, Perforating Company, product #738, tel (888) 278-6209. For approximately $275.00.

Poly-mesh: HDPE Poly-mesh liner, (for drying labware), catalog #09400-60, sold by the foot in 39" wide pieces. Cole-Parmer, P.O. Box 6690, Vernon Hills, IL 60061-6690, tel (847) 549-7600. $3.85 per foot.

G-10: Epoxy impregnated fiberglass computerboard. 0.010" x 36" x 43". Available from local plastic outlet store or Conservation Support Systems, catalog # PF- 18110. P.O. Box 91746, Santa Barbara, CA, tel (800) 482-6299. $24.23 a sheet.

Plexiglas: 1/8" clear acrylic sheeting. Local hardware or art store.

Pipe fitting: NIBCO G 4804, It is called 1-1/2" x 1-1/4" but has a 1-1/4" diameter threaded opening on one end with a pressure fit collar and a 2" diameter non-threaded opening on the other end. Available at your local hardware store for about $1.50.

C-clamp and Hose Clamp: Available at your local hardware store for about $3.50.

Shopvac^�: can be found at Home Depot for about $35.00

Shopvac^� 7' hose: catalog #90512, Accessory Dept., Shopvac^�, 2323 Research Rd., PO Box 337, Williamsport, PA 17701, tel. (717) 326-3557.

A Heap 'o HEPA Information

If you read the Health and Safety column (in this Newsletter and in the previous Newsletter), you might be looking for a HEPA vacuum. The following information on some of the leading brands of HEPA vacuums, complete with the following charts, should prove helpful with your search.

There are many vacuums on the market made specifically for lead clean up. These vacuums have within their filtration systems a filter known as HEPA, High Efficiency Particulate Air filter. HEPA filters have a minimum of 99.97% at 0.3 micron efficiency to meet OSHA standards. There is another, more expensive type of filter called ULPA, Ultra-Low Penetration Air filter, which is rated at 99.9995% at 0.12 microns. Similar filters can be found in allergy vacuums which poses the question, what is the difference between the two?

Allergy vacuums also come with a HEPA, ULPA, or HEPA-type filter (you might want to stick with HEPA or ULPA for conservation use). Allergy vacuums are made for use in the home and are thus usually constructed of plastic, rather than metal. But basically there is no difference between allergy and lead-specific vacuums since the HEPA filter is included in the filtration system of both types.

The Minuteman X829 has a HEPA filter which appears to meet the ULPA specification; the Ultivac/Particulate Vacuum meets the HEPA specification but at 0.12 (as opposed to 0.3) microns; Nilfisk offers the option of an ULPA filter for the GS80.

A word of caution, both HEPA and ULPA filters are very expensive to replace. The greater the number of filter stages, the less dirt will reach the HEPA filter and the less often you will have to replace it.

Speed control is another issue to consider. Only the Nilfisk GS80 offers a speed control option. It is possible to buy motor speed controllers separately and install them yourself, but they may not be compatible with the unit.

Comparative shopping will be beneficial. For example, tool kits are not always included in the price of the unit, but the kit usually isn1t too expensive. Be aware: many advertised prices are exclusively quoting the vacuum cost alone features, such as the HEPA filter, are an added cost. Good luck and happy HEPA hunting.

Comparison of HEPA/ULPA Vacuums

Information on Suppliers