There are many ways of making frit. I have used a variety of instruments to make frit. I am sure there are many more ways, but these are the ones with which I have experience.
The frit maker can be any of a number of things.
The mortar and pestle is a very good implement for small quantities of frit. It produces a variety of sizes with little contamination, especially if you use a ceramic set. You use a grinding motion mainly with occasional thumps. Cover the open mortar with a cloth to keep the bouncing glass pieces from escaping.
A coffee grinder whether hand cranked or electric is easier. But it tends to produce one size of frit in small quantities unless you control the grinding to short pulses and frequent sieving. The other drawback is that the blades of the electric grinders tend to be stainless steel which cannot be removed by magnets. Really thorough cleaning of frit is required.
I have used an adapted kitchen sink waste disposal unit. It produces frit of two sizes – powder and whatever the grid opening is. The results contain quite a bit of metal and need to be cleaned very well. The advantage is that it can produce large quantities of frit quickly.
The closed end pipe and plunger is much more work but can produce quantities of varied sizes of frit. It is better to use mild steel rather than stainless steel, as much of stainless is not magnetic, and so the metal fragments cannot be removed in the simplest way.
Shock treatment. This uses thermal shock of the glass to assist in the frit making process. It is described elsewhere.
Grading and Cleaning
Sieve all the fines and powder out of the frit and discard. The metal fragments mixed in these fines can be removed by multiple passes of a strong magnet.
See here for a method of keeping the magnet clean. However, there are often a number of contaminants that cannot be removed with a magnet. Washing powdered glass, is a good way to block drains. So the best and safest thing to do is to remove these fines from your frit. A kitchen sieve works well to this. You can dig this into your garden, as it makes a good soil conditioner, or bag it and put it into the municipal waste.
Grade the remaining frit with a garden riddle or other graduated set of screens. At this point you may wish to wash your frit to remove any powders from the glass. Do this in a basin to avoid eventually blocking your drains. The resulting water can be poured on the garden, or indoor plants. Alternatively, decant the water from the heavy glass powder and put the damp powder into a sealed bag to protect the refuse workers if you dispose of it in public waste.
You can dry the frit by leaving it in the open air and stirring it occasionally. You may wish to place some newspaper over the open tray of frit to keep it free of contaminants. This still has the risk of dust settling onto the frit as this method takes days to dry. The quicker alternative is to put the glass in flat open metal containers such as baking trays on top of the kiln. A bit of aluminium foil placed on top of the tray will reduce the dust that can get onto the frit.
Put resulting dried and graded frit in labelled containers for future use.