Often students will ask during the Traditional Water Gilding class whether the measurement of materials should be by weight or volume when preparing rabbit skin glue size, gesso, and clay bole. I usually preface my answer by first pointing out that I prefer the metric form of measuring as I find it more exacting.
When I switched many years ago from the US table of measurements to metric I performed a simple test: I measured by eye what appeared to be an ounce by volume of dry rabbit skin glue and then weighed it on a metric balance scale for comparison. Each time I performed the test the metric equivalent was different. That was convincing enough for me to make the switch to metric; I needed to match samples of gilded picture frame finishes as closely as possible and since there are so many things that can go wrong in gilding it's best to control what can be controlled and that would include the measurement and freshness of materials.
The question of whether to measure by volume or weight is a little more complicated. The equivalency of measuring materials by either of these two methods depends upon the material. Water may be measured either by weight or volume (1 ML of water = 1 gram). So which is better, to measure water by volume in a Graduate or weigh it on a metric scale? To answer this let me illustrate that in the past when I chose to weigh dry materials such as rabbit skin glue and whiting on a metric scale I measured water by volume in a Graduate, keeping the beaker as steady and even as possible for accuracy. However, I inadvertently discovered one day that a specific amount of water that I measured by sight of volume did not equal the same when weighed on the metric scale.
The reason for the discrepancy is due to what is called the meniscus which is the curve in the upper surface of the water in the Graduate. A meniscus may be either convex or concave depending upon the liquid and the surface material of the object containing the water. Water and the use of a glass beaker or plastic Graduate will produce a concave meniscus as shown in this photo of colored water in a burette, the proper reading being 20 ML:
What this means in practice for gilders is that the proper measurement of water requires viewing the surface of the water through the measuring container straight on so that the meniscus can be seen; it is the bottom of this curve that determines the proper measurement. As an example, if you are seeking to measure 50 ML of water make sure that the bottom of the curve is at the 50 ML mark.
To discover how much difference there might be by virtually 'eyeballing' a certain amount of water in a Graduate I tested measuring by volume 50 ML of water without specifically looking for the meniscus but still seeking a measurement that seemed fairly accurate. I then weighed this amount of water on a metric scale with a result of 41grams! When making small batches of rabbit skin glue size or bole mixtures this nearly 20% variance could prove quite drastic, partially because we run the risk of using a glue size that is stronger than we think.
What we can learn from this is that we need to seek the meniscus of water if choosing to measure by volume. Considering the chance for error, however, it is simpler and safer to weigh the water on a reliable metric scale.