Volume 16, Number 3, Sept 1994, p.11

Technical Exchange

by Dean Yoder, Column Editor

Watts Towers Mortars

In our work on the Watts Towers of Simon Rodia in Los Angeles, we use about 1 1/2 bags per month but have had several problems with Jahn mortar: inconsistency in the color of M70-18A and disbelief in the wide variation in compression values published for the Jahn injection mortars, M30/40, which we do not use. In a series of commercial lab tests on specimens made at our site, we were only able to achieve 64% of the published compression values for mortars M70-11, M70-18A and M90. The response from the distributor was that we did not use the proper amount of water. I do not agree with that explanation.

In 1990 and 1991, a local testing lab ran compression tests on three each, standard specimens of the Jahn M70-11, M70-18A, M90 and on a special cement mortar mixture. See the specs, below. The average compression values of the Jahn specimens were 2,280 psi, 36% lower than the value listed in the literature. (The artist's 40-70 year-old mortar, taken from a few selected areas tested at 3,000 psi, 4,000 psi and 9,000 psi, in the base area of one sculpture.) Because of the low value and to reduce unwanted failures in higher stresses repair areas, we have restricted use of Jahn mortar to non-structural members and those carrying low loads and stress levels. We use a cement mortar mix which has tested in compression at 2,700 psi (above Jahn but below Rodia's mortar) in structurally stressed members. This mortar is as follows: 1 pt. cement, 3 pt. sand, .1 to .25 pt lime

Aggregate: clean sharp natural sand conforming to the following size gradations and the requirements of ASTM C144

Sieve Size

percent passing




95 to 100


70 to 100


40 to 75


20 to 35


2 to 15


0 to 2

Water: Potable, free of oils, acids, alkalies and organic matter

N.J. Bud Goldstone
258 S. Rexford Dr., Beverly Hills CA

Sturgeon Glue for Consolidation

Also known as Isinglass, (from M Du: lit. sturgeon's bladder.)

Colorless, or milky white made of the swimming bladder of fish. The best grade is "Russian isinglass", prepared from the internal membranes of sturgeon's bladder. Thanks to its great tacking properties and low viscosity sturgeon glue has been used widely in Europe as consolidant for paintings.

Preparation method:

To make a solution of 7-10% use weight / volume ratio of dry glue to water.

After weighing it, place a small amount of dry glue pieces in a dish in just enough distilled water to cover it. Allow to soak for several hours or overnight, until glue turns grayish and soft. You may then knead the soft glue into a uniform mass. Add the remaining distilled water to cover it and heat slowly on a double boiler. Do not overheat, ( 140-150 F ). Overheated glue will lose some of its excellent tacking properties. Stir until glue is just dissolved. It may then be strained, prior to use. A preservative can be added but it is best to use a fresh solution each time. I found that it may also be refrigerated for a few days, without a preservative.

It may be used alone or immediately preceded by a mixture of ethyl alcohol/ water 1:1, to decrease surface tension. In cases of severe cupping, relaxation of hard paint flakes can be achieved through solvent vapor treatment, i.e. local "solvent vapor chambers" prior to consolidation. The warm glue solution can be brushed under the cupping flakes, or indirectly through Japanese tissue. Warm electric spatula can be used to relax the infused areas of paint to re-adhere the cupping paint flakes. The whole area is left to dry under minimal pressure.

For other references see also: T. Petukhova and S. Bonadies "Sturgeon glue for painting consolidation in Russia", Journal of AIC, vol. 32, number 1, 1993.

A. Zebala
Paintings conservator.

User-friendly Technical References

Museum staff often ask for user-friendly technical references. CCI Notes published by the Canadian Conservation Institute offer practical explanations and suggestions on a variety of topics. Topics of particular interest include collection care, museum environment, spot testing, and disaster planning.

Each article is accompanied by a bibliography for further reading. If further explanation is needed, every article includes the name and telephone number of the author.

The articles can be requested individually or in a ring binder. Contact CCI at 613/998-3721 or at 1030 Innes Road, Ottawa, Canada KIA OC8.

Shelly Reisman Paine
Objects Conservator
Nashville, Tennessee

Heat Resistant Tape for Infiltration

Method of Glass Conservation The basic method for gluing glass, called infiltration, employs epoxy resins of low viscosity and uses strips of adhesive tape for the temporary alignment of broken glass. This method is described in Conservation of Glass by Sandra Davison and Roy Newton, Butterworth 1989.

This method can be altered by warming up the glass and the prepared epoxy-resin and replacing the regular tape with Poly-Tape (used for masking slides). Warming does two things:

  1. It significantly lowers the viscosity of the epoxy, thereby enhancing penetration into the cracks. This technique is especially useful for thick-walled glasses.

  2. It speeds up the setting process of the resin. The correlation between applied heat and setting time is described for the epoxy HXTAL-NYL-1 in the catalogue of Conservation Materials, Ltd.

When heating the epoxy (do not overheat), a hot-plate can be used to control the temperature. An ordinary IR-lamp is useful for heating the glass (the glass should only warm to the touch). The aluminum of the Poly-Tape reflects the irradiation so that the pressure-sensitive layer beneath does not become soft. The alignment of the broken glass stays secure.

For accelerated setting, the IR-lamp can be left on the glass after the resin has been applied. Instead of a five-day curing period at room temperature, only several hours are needed to achieve a proper setting.

Tweezers are used to remove the Poly-Tape and the excess resin can be removed safely with a scalpel or acetone.

For information about 425 Met Poly-Tape 1/2" x 100", contact:

Horizon Tape Products Company
251 LaFayette Frontage Road
St. Paul, Minnesota 55107
612 224-4083
Rainer Richter
Mellon Fellow, Objects Conservation Cleveland Museum of Art

Micro-Heated Spatulas

Have you ever been frustrated by the size and shape of your heated spatula? Michael Becker, a jeweler in Munich, has been manufacturing micro-heated spatula tips for conservators in Germany.

Rainer Richter, Mellon Fellow in Objects Conservation at the Cleveland Museum of Art, has been using these micro-tips for molding wax fills in medieval enamels. The tips can be made of "Fein" or sterling silver; "Fein" gold tips are also available, as the silver tip, which contains 75/1000 copper, can cause discoloration of certain waxes. The spatula tips come in a variety of sizes, ranging from 2 x 4 mm (about the size of a grain of rice) to 6 x 12 mm. Different shapes or materials can be made upon request.

These tips are ideal for setting down miniscule tenting, blistering, and cleaving paint layers.

The tips are designed to slip over a small, light weight micro-heating needle. These heating needles are actually miniature soldering irons for pin-point soldering of micro-circuits. The needle requires a 6V power source with a rheostat to operate.

If your present heated spatula has interchangeable tips, an adaptor can be fabricated to accomodate these micro-spatulas.

All of the components of the spatula are available in Germany, although they may have a supplier in the United States.

To order the spatula tips, contact:

Michael Becker
Artelerie Str. 6
D - 80636 Munchen
Tel.: (011 49 89) 1235703
Fax: (011 49 89) 398077

The specifications for the heating needle are as follows: MinorMinityp Lotgerat Minor 6V, Ersa Company. It can be found in most German electronic stores, such as Burklin Die ganze Elektronik.

Dr. Hans Burkin
Schillerstrasse 40
80336 Munchen
Tel. (011-49 89) 5 58 75-0
Fax (011-49 89) 55 53 23
Dean Yoder

Cold Molding Compounds

Cold molding compounds are available from many sources, but I have found the best supplier to be the Perma-Flex Mold Company, 1919 E. Livingston Ave., Columbus, OH 43209 (orders 614/252-8035). They offer a technical information line (614/252-8034) and are always helpful and pleasant whether the questions are of a general nature or about a specific problem. Their product list is broad, consisting of both in-house formulations and those by other manufacturers.

Most of my experience with RTV silicones has been with wax pattern making for the lost wax process, although they have many other uses including resin casting. Silicone rubbers give the best waxes because the surfaces are dry and easy to read. The products I use most are BLU-SIL (Perma-Flex) and V-1065 (Loctite). BLU-SIL is very easy to pour and deairs readily. It is soft, flexible, and has good dimensional stability. One disadvantage is that it is fragile. Deep undercuts or projections into the interior of the mold may tear. A mold release application before each use will help.

V-1065 is an excellent product for many situations. Its properties can be adjusted to suit your needs. Even a quick-cure catalyst can be added to allow brush applications on vertical surfaces. It has a high viscosity typical of most silicones and deairs slowly. If you do not have vacuum capability, an initial brush coat (before pouring the main body of the mold) will ensure a good surface.

Polyurethanes are another class of cold molding compound. Perma-Flex P.U.R.E. 1040 Green is what I prefer. Its use in wax pattern production is limited to large work because of the tendency to produce tacky waxes. However, it is good for the plaster and ceramic slurry work for which it was designed. 1040 Green is easy to measure, easy to mix, and pours freely with good deairation. It is tough, long-lasting, and less expensive than silicones.

All cold molding compounds undergo some long-term changes. It is wise to discuss your specific use and mold life expectation with the dealer. Products will vary in toxicity so gloves and ventilation are always recommended. A good practice is to clean up after each use and to keep containers free of spillage.

Jim Cutrone
Cutrone Casting and Restoration
216/ 621-4448

23 Karat Gold Powder

23 Karat gold powder is useful in restoring losses in worn gilt surfaces. The gold is applied to the (filled) area of loss with a small amount of binder, such as Kolner aqueous size and/or watercolors. After it is dry, the gold may be burnished to varying degrees with cotton or an agate stone.

The gold powder has many advantages: its ability to conceal blemishes (compared to mica powders); its stability (compared to bronze powders); and it can be thinly applied and burnished to imitate worn gilt surfaces. Silver powder is also available, but, of course, it lacks the inert quality of gold. Both are available from Conservation Support Systems (tel 805/682-9843 or fax 682-2064) as well as other suppliers.

Matt Fleischman, Objects Conservator
Carnegie Museum of Art


Inexpensive weights for conservation treatments can be obtained through your local diving and scuba stores. There are hard and soft weights. The hard weights are forms of lead covered with dense plastic. The soft weights consist of lead shot covered with waterproofed synthetic fabric (as is used in camping and hiking equipment). The weight covers did not bleed or show any signs of distortion when tested with hexanes, heptane, ethanol, acetone, methyl ethyl ketone, toluene, and xylenes. The weights are available in 2,3,4,or 5 lb. sizes and cost between $2.00-$2.50/ lb. Judy Walsh of the NGA brought the hard scuba weights to my attention.

Stephanie Watkins

Tag Sale

A tag sale open to the public will take place in the old SFMOMA building at 401 Van Ness Avenue, SF, on Oct. 1. Items to be sold include desks, chairs, tables, file cabinets, etc. Call 415/357-4125 for more information.

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