To make 1/2 pint of size use 2 to 3, 1/2" squares. For one pint, use 4 to 6 squares. A weak mixture can result on the gold loosening or peeling from the glass. An overly strong mixture can make it hard to remove excess leaf, and could become hazy with time.
Soak the squares in clean water for 20-30 minutes. Distilled water is recommended, as contaminants may cloud the gelatin size.
Heat the mixture slowly, and stir with a clean utensil, until the gelatin has completely dissolved. DO NOT BOIL, as this will degrade the gelatin size.
While still hot, the mixture can be filtered through a fine sieve to eliminate any un-dissolved gelatin.
Remove from heat and allow to cool slightly before use. Gelatin size works best when warm.
Optional: To help break the surface tension of the size, so that it will totally wet the surface of the glass, some gilders add one drop of dish washing liquid for every half pint of gelatin size.