May 1999 Volume 21 Number 2

Technical Exchange

WHAT'S NEW ABOUT FILLING, NYET!

Using sturgeon glue as a component in consolidation, lining, and filling has a long history in Russia. It is the traditional method and is still widespread; students are trained primarily in sturgeon glue based fills (with wax-resin fills in certain situations). These methods have been proven over time and are well trusted.

In Russia, it is common to consolidate the entire surface of a flaking or cupping painting with sturgeon glue. If this is not done prior to filling, 5% glue sizing with honey may be applied before filling. Because the glue is in such low concentration, it penetrates well into the canvas, ground and paint layers. Because of this, sturgeon glue fills seem to hold to surfaces better than proprietary acrylic compounds. It is common practice to varnish after filling to improve the bond and the continuity between the fill and the adjacent paint layers.

One technique to enhance flexibility is the addition of honey in a 1:1 ratio to the dry sturgeon glue. Sturgeon glue based fills are also used on icons or frames, though without honey, because rigid supports generally don't require as much flexibility.

Preparation of Sturgeon Glue

We have tried commercial products that are in sheets or threads, and have found that these pre-made, processed products have less adhesion than freshly-prepared glue from the dried air bladders.

A detailed description of how to prepare sturgeon glue from the bladders is set out in a Technical Exchange column in the WAAC Newsletter, Volume 16, Number 3 (1994) by Aneta Zebala. Aneta suggests preparing a stock solution of 7-10% w/v ratio of dry glue to water. We prefer a 3%-5% w/v ratio for consolidation, depending upon the condition of the paint layer and its thickness. Usually, we use a 3% solution for facing, and a 5% solution for consolidation and as a base for fills. It is beneficial to prepare a stock solution of 20%-30% and dilute with water depending on your needs, because stronger solutions can be stored in a refrigerator for up to a couple months.

A characteristic of the glue is that the concentration can be tested with a finger tack test. A small amount of glue is rubbed between the thumb and index finger. With experience, one can determine the strength of the glue by quality of the tack. If you begin to notice thin, wispy strands forming between your fingers, the solution is probably 8% or stronger. This technique is helpful when you are using the glue over a course of a day, and a certain amount of water may have evaporated, creating a stronger solution than you started with.

We often prepare sturgeon glue in advance: cook it in a double boiler without honey with a minimum of water at temperatures around 140 or 150 degrees Fahrenheit (as per Zebala's instructions), pour it out on a sheet of clean Mylar, and let it air-dry naturally. The thin sheet of dried sturgeon glue can be cut with scissors and stored in a jar for years. It can be easily weighed and redissolved in distilled water in a double boiler. This method also allows you to control the concentration of the sturgeon glue solution more precisely.

Preparation of Fill Material

A basic preparation is a solution of 5% sturgeon glue with honey (95ml water, 5g dehydrated sturgeon glue, 5g honey) mixed with chalk to the consistency of sour cream. You should always work with a warm fill material. For small losses, the chalk and glue can be mixed directly in the palm of the hand. Your hand temperature will keep sturgeon glue consistently warm so you won't need to reheat it.

Textured Fills

In our experience, sturgeon glue based fills seem to accept texture better than proprietary polyvinyl alcohol-based filling compounds. Our technique for achieving a textured fill is as follows: First level and smooth fill with slightly damp cork. A canvas similar in weave to the support canvas is chosen, and then dampened (saturated then wrung out as much as possible). If the canvas is too wet the fill may be dissolved. Then imprint the texture of the dampened canvas into the fill with a warm iron (140-150 degrees Fahrenheit). Although this technique produces a negative imprint, and may not be suitable in all cases, it still produces a very acceptable surface. If it is necessary to remove chalk residue from around the fill on the paint surface, we sometimes use a bit of baby (pH neutral) soap, if the paint layer permits it.

(Elena King is a graduate of the Academy of Art, Conservation Department, St. Petersburg, Russia.) For more info. call: (216) 231-7880 or Email: yodercon@aol.com