The first time I packed frit into a mold I learned the difference between the volume of frit and the FIRED volume of frit.
Even so, when you’re casting or fusing with frit, the level of shrinkage can be disconcerting.
Here is an example. Fill up a test mold to the brim with powder, tamp it down tightly so that it looks like the first picture (on the right):
Now, fire it in a full, casting fuse. It will come out of the kiln looking like the mold below. The cooled glass will be a bit less than half of the original volume:
Blame it on air. Literally.
What’s happening is pretty basic: The smaller the particle of glass, the more particles you can pack into a volume, the more gaps you have between those particles, and so the more air.
Fire the frit and some of the air is trapped in the resulting glass, making bubbles. Most of it, though, is squeezed out as the glass softens, leaving you with mostly glass instead of glass+air.
Without all the air, the total volume decreases, sometimes significantly. Unfired, a solid, bubble-free chunk of glass will always take up a lot less volume than its gritty cousins.
Below, I’ve weighed out 1.5 ounces (about 43 grams) of glass in everything from glass powder to billet. Look at the difference in volume:
The powder takes up about three-quarters of the cup, while the billet barely splashes the bottom.
Moral of the story? For best results, measure frit by weight, not volume.
When I want frit to cover an area completely, to a precise depth, I cut a sheet of scrap glass to the right size, set it in place to make sure it fits, then weigh it.
I weigh out an equivalent amount of frit–that much needs to be packed into that space to achieve the thickness I need.
Or I do it without the glass: Measure the area, take its volume (LxWxH) and multiply it by the specific gravity of the glass (2.5). For example, if you want to fill a space 5 cm long by 7.5cm wide by .6cm deep (about 2 x 3 x 0.25 inches):
Volume: 5 x 7.5 x 0.6 = 22.5 cubic cm
Weight of glass: 22.5 x 2.5 = 56.25 grams of glass
(or about 2 ounces of glass)
I use the metric system because it’s a bit more precise when dealing with small amounts. But either method works consistently well for me.