Q. What would be the reason to choose either oil size or acrylic emulsion when using a method of gilding other than traditional water gilding?
A. Oil size and acrylic emulsion, sold under such names as Water Based Gilding Size, Aquasize, or Wunda Size, are a form of mordant gilding. Mordant gilding employs the use of an adhesive to adhere gold, silver, or imitation leafs to a prepared surface. Other mordants include gum tragacanth, gum ammoniac, gum arabic, garlic, and glair (beaten egg white).
The use of oil size, a form of mordant gilding, is commonly referred to simply as oil gilding. It's a specially formulated varnish which produces a satin to semi-gloss tone in the leaf. It does not produce the high luster of traditional water gilding but it has an attractive appearance of its own. It offers a durable finish and is used for both interior and exterior gilding projects.
Oil size is self-leveling whereby the oil tends to smooth out after it's brushed on thinly. It cleans up with mineral spirits and so is somewhat messy to work with so be prepared to use rubber gloves, spirits, and perhaps some acetone in the cleaning of your brushes.
Oil size comes in different drying times ranging from 3 hour to 24 hour. In practice however, once a can has been opened the drying time becomes shorter and shorter. It's best to open the can, stir the size to bring the driers to the surface, and pour some into another container for use on your project. This way you can close the can of oil size and preserve its life.
The water-based sizes are much cleaner to work with as they only need soap and water for cleanup; just make sure you clean your brush once you're done as the size will harden and will require acetone to remove the dried adhesive. One of the benefits of the acrylic emulsions is that once it's applied to a surface it will come to the proper tack in about 15 minutes and remain properly open to gild for about 36 hours. The drawback is that it never seems to quite dry. A clear coat after 24 hours can help protect the gilding from this effect.
Acrylic emulsions have a brighter appearance than oil size but do not have the brilliancy that you can only get from traditional water gilding nor does it share its almost magical tone and glow. As for which size to use, oil or water based, it's important to know that most any high quality project which is not water gilded uses an oil size. That said, acrylic emulsions can provide a gilding option that is both quick and much easier for the beginner.