If you make your own fine frit and powder, make sure it is clean to avoid black specks, or a grey appearance caused by metal dust and fragments.
Clean the glass you are going to break up before you start the process.
Use mild steel or other magnetic metal to break up the glass, or protect the glass from the breaking tools with layers of paper, plastic, cloth or combinations of these materials.
Then with a powerful magnet remove any metal residue from the frit and powder. The magnet will need to be passed over and through the glass particles a number of times, cleaning the magnet after each pass. To ease the cleaning you may wish to put the magnet in a plastic bag. Then move the bag over the waste bin and remove the magnet. The particles fall into the bin.
Do not use stainless steel to break up the glass as it will not be attracted to the magnet. Stainless steel particles will result in the same discolouration as if you left the glass uncleaned.