Formulas for varnishes and mediums call for measurements that are often confusing for artists. For example, in his formula for a glazing medium, Ralph Mayer calls for a “5-pound cut” of dammar (or damar) varnish.1 This measurement refers to the amount of dry dammar resin dissolved in spirits of gum turpentine in a ratio called a “cut,” which refers to the amount of resin in pounds dissolved in one gallon of solvent. This measurement is simple enough to use when preparing your own varnish, but what if you want to use a commercially-prepared dammar varnish, such as Rublev Colours Dammar Picture Varnish or Neil’s Best Damar Varnish? How do you know what "cut" it is?
You will notice in the information provided for Rublev Colours varnishes that the resin amount is specified as a percentage, such as 31% (w/v), where w represents weight and v is volume. This is a common scientific expression to describe the amount of solute2 in a solution. This expression measures the amount of solute in grams and measures the amount of solution in milliliters. In our example, Natural Pigments Neil’s Best Dammar Varnish is a 31% (w/v) solution. It contains 31 grams of dammar resin for every 100 mL of varnish.
|weight of solute (in grams)
|Volume percent =
|volume of solution (in milliliters)
Because of the different units in the numerator and denominator, this type of concentration is not a true percentage. It is used as a quick and easy measurement because volumes are easier to measure than weights and because the density of dilute solutions is generally close to one gram per milliliter (1 g/mL). Thus, the volume of a solution in milliliters is very nearly numerically equal to the mass of the solution in grams.
What is a 31% solution in “cut” measurements? Neil’s Best Dammar Varnish contains 31 grams of dammar resin for every 100 milliliters of varnish. This amount of dammar equals 1.09349282 ounces, and 100 mL of solution is 3.38140227 U.S. fluid ounces. If we multiply the solution volume to equal one U.S. gallon and use the same multiplier for the weight of the dammar, we obtain 41.39 ounces or 2.59 pounds. This means that a 31% solution is almost equivalent to a 2.6-pound cut of varnish. To use Neil’s Best Dammar Varnish in Ralph Mayer’s glazing medium formula, we double the amount of varnish required and reduce the amount of the turpentine from 5 fluid ounces to 4 fluid ounces to obtain the same results. Hence the formula would read:
|1 fluid ounce
|2.6-pound cut) 2 fluid ounce
|4 fluid ounces
1. Mayer, Ralph. The Painter’s Craft. An Introduction to Artist's Methods and Materials. Revised and updated by Steven Sheehan, Director of the Ralph Mayer Center, Yale University School of Art. New York: Penguin Group. 1948. 1991. p. 110–111. The formula for the glazing medium is as follows:
Stand Oil 1 fluid ounce
Dammar varnish (5-pound cut) 1 fluid ounce
Pure gum spirits of turpentine 5 fluid ounces
Cobalt drier 15 drops