September 1998 Volume 20 Number 3

Technical Exchange

Old World Oxgall for Removing Oil-Resin Varnishes

Oxgall is the bile obtained from the gall bladder of a bovine animal, usually a domesticated cow. It consists of a ropy, mucous, semi-transparent liquid, and contains sodium salts of taurocholic, glycocholic and deoxycholic acids, as well as cholesterol, lecithin, etc. In Russia, oxgall is available in drugstores as it is often used as a medicine for treating arthritis and bruises.

Until recently, in our studio, the only use for oxgall has been as an wetting agent for gouache or watercolor to reduce surface tension during inpainting. Elena Valentinovna King, a recent member of our staff, has been showing us some Russian techniques that she learned during her training at the conservation department of the St. Petersburg Academy of Art, Repin Institute of Art, Sculpture, and Architecture.

Oxgall-alcohol-water solutions are commonly used to remove stubborn grime coatings from the surface of icons and traditional easel paintings. It is also possible to thin an oil-varnish layer with an oxgall-alcohol solution, though we wonder how much residual oxgall is left behind. Russian conservators always wash the surface of the thinned varnish to remove oxgall residue.

We've been recently experimenting with an oxgall solution (purchased from Talas) to remove varnishes that contain large amounts of drying oil, which are normally insoluble in alcohols or acetone and may require the use of solvents such as 1-methyl 2-pyrrolidinone. In one instance, we used a poultice containing equal parts of oxgall, isopropyl alcohol, and water to gel the varnish layer. Next, cotton swabs (dipped in acetone) were lightly rolled over the gelled varnish; this allowed us to dry the surface and remove any residue. In this case, we think that the alcohol helped to penetrate into the varnish structure, allowing the oxgall and the water to swell the varnish layer.

So far, we've successfully removed an oil-varnish layer from two 19th century paintings by the English painter, John Russell, with the oxgall-isopropyl alcohol-water poultice. After cleaning and rinsing with water, the paint surface had a desirable subtle sheen when viewed at 40x. We feel that this oxgall solution was much safer than the 1-methyl 2-pyrrolidinone, however one should take into consideration the exposure time and solvents used to clear the surface.

Follow-up on Slide Mounts

To add to my previous contribution on slide mounts: The newest version of the ANSI enclosures standard, (American National Standard for Imaging Materials - Photographic Processed Films, Plates, and Papers - Filing Enclosures and Storage Containers; ANSI/PIMA IT9.2-1998), now available from ANSI, has been modified to include polystyrene in its list of acceptable plastics for use as enclosure materials. The change was specifically directed towards slide mounts, the most common application of a polystyrene enclosure.

Treating Tinplate

From a DistList posting concerning the corrosion of tinwares,June 26:

Tinplate is a material that was formed by first pickling wrought iron sheets in acid to remove fire scale, dipping the sheets in tallow or palm oil, and then finally heating the prepared sheet iron to a cherry red color and immersing them in several baths of molten tin. The tinplate that is used for architectural purposes is actually a lead/tin alloy that is resistant to the weather and is called terne plate.

Tinplate corrodes because micropores are present in the tin coating. Tin is a more noble metal than iron and thus the iron corrodes at the base of the pores. The iron salts are carried by an electrolyte (moisture in the form of RH) and forms a hard, expanded corrosion crust along with ferrous staining of the surface. Often the tinned surface lies intact underneath the rust. The reason why so many kitchen items are tinned, both ferrous and copper alloys, is that tin salts are non-toxic. Often the tinning was applied to just the interiors of kitchen wares. The reason why tin cans don't rust on their interiors is because tin ions are complexed by the food products and this greatly reduces ionic activity. There is a reverse in polarity of the tin in an active direction. Thus, on the interior of the tin can the tin is actually anodic and is sacrificial and the iron substrate becomes cathodic and is protected.

The solder joins, usually a lead based solder, are usually stable because of the less noble nature of ferrous metal.

So, my best advise (to the historical society who posted the query) would be to reduce the exposure of these objects to relative humidity that drives electrochemical corrosion. Get a conservator out to survey the objects and to make specific recommendations. Do not try to clean these objects yourselves (especially if any of these have further surface treatments such a paint).

I treated about one hundred objects for a kitchen exhibit several years ago, the majority of them tin plate, and they were among some of the toughest, nastiest, treatments that I've encountered. (ie., a raised, hard, steely ferrous corrosion crust with a soft tin surface underneath, mingled with layers of historic cooking grease) Yuck, these artifacts did stink!"

Materials for Soft Weights

On May 5, when the WAAC toured the labs at the Getty, several people were interested in the materials we chose for the soft weights used in the book & paper lab. Here is a description.

The weights are filled with garnet that is #8 size pebbles (about the size of split peas). A 100 lb bag costs $26.63 (described as ECGMCO on the invoice) and is the minimum amount that can be purchased from Gordon Sand, P.O. Box 5549, Compton 90221. They are located at 2201 S. Santa Fe Ave. and the phone# is 213-774-7930.

The garnet is rinsed prior to filling the lining sack which is made of airplane linen from Bookmakers (6001 66th Ave, Ste 101, Riverdale, MD, 20737) The lining is stitched closed and then put into a cover that is 1/8" larger all around (or, in length and width for the "snakes") than the lining. A neutral color cotton or silk velvet is nice for using in the Reading Room but we are converting to using the airplane linen as the covering material for use in the lab.

and more

Object Stabilization Bags

Flexible and loosely packed, Object Stabilization Bags No 2. contain minute gauge sandblasting media. This media contains no salts, no free silicas and is 100% inert by CAL OSHA MSDS spec sheets. Bella Fine Art's OS No. 2 bags provide simple and consistent protection for a variety of uses from the weighting down of ephemera to the stabilizing of rare 3-D objects. The sand media is first sewn-sealed within an inner Nylon bag, then the bags are covered with an outer sleeve of 100% pre-washed cotton pinwale corduroy for a fine soft surface.

Prices:

Sizes and prices are examples. Custom sizes are available. Minimum order of 50 bags (any size) and 50% deposit required. Available from: Bella Fine Art, Tony Bellaver, 2241 Quesada Ave., San Francisco, CA 94124, (415) 642-7243.

A Query about Cellulose Acetate

Has any conservator successfully treated an object that was made from cellulose acetate which at the time of treatment was highly degraded? The degree of degradation includes extensive cracking, warping, outgasing of acetic acid, changing color and delaminating. If anyone has been successful, I would welcome your comments.

Another HEPA Vacuum

The September 1997 issue of the Newsletter contained a Technical Exchange item featuring a comparison chart (p.14) of 24 HEPA vacuums on the market, written by Batyah Shtrum. Since that time one of the manufacturers, Fantom Technologies, has introduced a new model which I recently purchased. It has passed muster with our HEPA expert, Chris Stavroudis, and because of its many good features, especially price, deserves a mention.

The vacuum is the Fantom Lightning (see it even has a jazzy name), a canister model which operates with a "dual cyclonic" action, which means that the dirt swirls around in a collection bin rather than being caught in a bag. ( It also means that the suction power doesn't diminish as the bag fills up, and you never have to buy bags.) When the bin is full, you remove it - it has a locking lid - and empty the dirt. There is probably slightly more risk of exposure at this point than if you just removed a bag, so you do have to exercise reasonable care. The bin is washable, and see-through so you can tell when it's full, and monitor the dirt that you are picking up.

The specs, for comparison with the chart are: 38 CFM; 80-90" water lift; .7 gal. tank size; dry vac only; 99.97/0.3%/micron filter efficiency; weighs 15 lb.; 1.7 hp; 67 dB noise level; 2 stage filter; approx. $330.

It can be purchased directly from the manufacturer, or at stores such as Costco and Sears (where it has a Kenmore name). I got mine on sale at K Mart.

Due to its "cyclonic action", the vacuum cannot be used with a motor speed controller. It does have an on/off air release valve on the handle which reduces the suction to a fairly gentle level. One would have to devise an additional air bleed if more control were necessary.

The unit is made for home use and comes with a powerful adjustable rotary brush for floors, with its own conveniently located on/off switch, and an onboard tool kit. The main on/off switch is very easy to operate and there is a retractable cord.