Frit can be created by thermal shock. You will still need to do some manual breaking up. The principle is that you heat the glass and then cool it rapidly, causing the glass to break into pieces.
Place the glass in a stainless steel bowl and heat as fast as possible to 300C – 400C. Turn the kiln off and pull out the bowl, using heat resistant gloves and dump the hot glass into a large bucket of water. Once the glass is cool, pour off the water and dry the glass. When dry, you can break the crazed glass into smaller bits just as you would with other glass. Note that pouring water over the glass has two disadvantages – one, it does not completely thermal shock the glass, and two, the large amount of steam released is very dangerous.
The advantages of this quenching method of obtaining frit are that you can create frit with less effort. You also get less fines and powder with this method. And less effort is required to smash up the glass.
Some indicate that ice cold water to quench the glass is a good idea. This is because warm water will not provide enough of a shock to the glass to craze it throughout. But if you have a large bucket of water, there is no necessity, as the volume of water will cool the glass quickly enough. Of course, if you are planning another quenching, you need to renew the water, as it will not be cold enough to thoroughly craze the glass.
You can, in part, control the size of the resulting frit. Firing at 300C results in larger frit than firing at 400C. However, firing at 500C does not provide even smaller frit. The best results are between 300-400C, although frit can be made at 200C as well. Experiment with temperatures to get the frit you want.
Once you have dried the frit, you can begin to break it up. Some can be done by hand, but the pieces are often sharp, so gloves are essential. The other standard methods of breaking up glass to make frit are applicable. But it does not take as much effort as breaking from cullett.