The process of gilding wood involves the use of either of two methods: traditional water gilding and oil gilding. Sometimes the two methods are combined on the same object for different effects. However, if we oil gild with an oil size, what are we doing then when we gild with a water based acrylic emulsion? Good question!
The modern day acrylic emulsions for use in gilding - Wunda Size or other such products - are water based but are certainly not to be confused with traditional water gilding, a centuries old, complex and beautiful method of gilding with genuine gold leaf. Since acrylic emulsion sizes are not oil based they cannot be categorized as oil gilding. So what are they?
Oil gilding uses a linseed oil-based varnish-like medium which we call oil size. The term size implies an adhesive. Oil sizes are referred to according to their drying times: slow size and quick size. Within these two parameters there are actually a number of different drying times available - usually 1 hour, 3 hour, and 12 hour. The hour designation refers to the approximate length of time it takes for the size to reach the appropriate tack window needed to begin gilding. This window of time, where the size is just tacky enough that when you draw your bare knuckle across the oiled surface you hear a squeak, eventually closes to the point where the oil will no longer adhere the leaf to the surface.
A slow dry size, a term that is reserved for the 12 hour version, has the longest open window of any of the oil sizes and has the greatest leveling properties. It takes a fairly long time to reach the window when one can begin gilding - approximately 12 to 17 hours - and remains open for quite some time after that, perhaps 36 hours or more depending upon weather conditions. Lefranc slow dry is preferred by many gilders and a new lead-free version is now available in the 12 hour version only.
A quick size comes to tack quickly but will leave its window quickly as well. It usually stays tacky enough, however, to do a small job nicely. I often mix a slow and a fast dry Rolco brand size at a 50:50 ratio to gain greater control over the drying times.
So with all this talk about oil size, where are we with acrylic emulsions? After applying the emulsion size to the surface, it behaves like oil in that it needs to have time to set up and a window that eventually (emphasis on eventually!) closes. But it's not oil gilding. It's water-based but it's not water gilding.
The explanation is actually quite easy. There is an umbrella term that covers both oil gilding and acrylic emulsions: Mordant Gilding. A mordant is an adhesive medium that is used as a binder for gold leaf as well as silver and other metal leafs. In fact, oil size and acrylic emulsions are only two mordants; historically, a variety of binders, or mordants, have been used in gilding including garlic, glair (egg white), gum ammoniac, and tragacanth.
The new acrylic emulsions have some positives and negatives. They are water-based, therefore clean up easily with soap and water allowing you to avoid the somewhat messy mineral spirits and gummy residue associated with oil size. However, oil size, especially the slow dry, is nicely self-leveling, whereby an acrylic emulsion essentially lays the same way that it's brushed onto the surface; it just sort of lays there, so you need to finesse it carefully by feathering it out. But the acrylics can be gilded after about fifteen minutes or as long as thirty-six hours. I tested a piece recently and gilded a board after the emulsion size had dried for thirty-six hours and it gilded very well with great retention. I wouldn't be surprised if the window stayed open for a week, which isn't actually a plus unless you're gilding some very large walls!
The one thing that bothers me about the acrylic emulsion are the brush marks left in the size since it isn't self-leveling. But it's certainly worth working with if you have a good touch and are good with a brush. Be careful though on the type of project you use it on because it never seems to ever really dry. If it's not a piece that's going to be handled and is given a light shellac coat, then by all means try it and see if you can use it in your own work.
For those of you interested in exploring oil sizes further, visit http://www.theletterheads.com/glawson/goldsize.html. The late Rick Glawson, a gilding colleague, was always full of helpful gilding wisdom.
See you next time...