Open time is how long size remains in the state where it can successfully accept gold leaf. It starts when the size is applied. It ends when the size has either dried too much or is still too wet. The duration varies depending on the size formula, temperature, and humidity.
Traditional animal-based sizes have a relatively short open time—typically 15 to 45 minutes depending on conditions. This requires skill and confidence. You must work quickly and accurately. You cannot hesitate or second-guess your placement. For experienced gilders, this speed and decisiveness is part of the craft. For those learning or working at scale, it is a limitation.
Synthetic sizes like Permacoll extend open time to several hours. This changes the work rhythm entirely. You can apply size to a larger area and work your way through it methodically. You can apply gold to one section while waiting for another section’s size to reach proper tackiness. This is not laziness—it is practical technique. The choice of size (and thus open time) determines the work pace and the scale of work that is practical. Understanding this is essential for planning a gilding project.