People setting out to tumble glass generally have one of two goals -- to smooth and polish their pieces of glass to a high sheen, or to recreate the frosted matte finish so prized in seaglass. Whichever outcome you aim for, there are a few things to keep in mind in working with glass:
Safety: Broken glass is sharp! Handle with care, preferably with gloves. If you decide to break the glass yourself, put the glass objects in a heavy fabric bag (canvas is a good choice) before wielding your hammer, and wear both safety goggles and gloves.
Out-gassing: Glass -- whether manmade or natural (obsidian) -- produces gas during the process. Check your barrel at least daily, and "burp" it if it shows any sign of swelling. You may also try adding a small amount of baking soda to your mix to minimize the gas production, though not during any final polishing step.
Cushioning your glass: Glass chips easily, so you should add something to your load to slow the process down. You can use plastic pellets or a variety of other materials to do this, such as: corn syrup, cornstarch, sugar, molasses, rice hulls, gelatin, sawdust, and even antifreeze (dispose of this last carefully, as it is poisonous when ingested). You will need to experiment to find the right level, as too much thickener will make each step take longer than necessary.
Don't mix the glass with other material, particularly stones that are harder than the glass (which is about Mohs 5).
If you have a rotary tumbler and want a high polish on your finished glass, follow the directions for tumbling stones with these changes:
Step One: If your glass is rough (as obsidian may be) or needs a lot of shaping, start with the first step, using 120/220 grit silicon carbide. Add plastic pellets or one of the other cushioning agents listed above. Be careful that the load level is right for your barrel. If, however, your glass is smoother, you may be able to skip this step and go on to Step Two. Check the progress every day or two, which incidentally will relieve any gas build-up in your barrel. Wash your glass and the barrel thoroughly when the step is finished.
DON'T WASH THE SLURRY DOWN THE DRAIN!
Step Two: Use 320 grit silicon carbide and a cushioning agent. Be careful with your load level. If it's getting too low, add (fresh or recycled from an earlier Step Two) plastic pellets. Check the progress and wash your glass and barrel as above.
Step Three (Pre-Polish): Use 600 grit silicon carbide and a cushioning agent. Be careful with your load level. Check the progress and wash your glass and barrel as above.
Step Four (Second Pre-Polish): Don't skip this step with glass! Use 600 grit aluminum oxide, rather than silicon carbide, as the different characteristics of the media will smooth scratches left by the silicon carbide. Use a cushioning agent, bring your load up to the proper level, and check progress and wash the glass and barrel as above.
Step Five (Polish): Use Cerium Oxide or Chrome Oxide polish. Use a cushioning agent, check your load level, and check progress as above. If your glass still has a slight haze, burnish following the directions for tumbling stones.
If you are using a vibratory tumbler, keep in mind that not as much rounding of the glass will occur. If your pieces are a lot more jagged in shape than you like, you may want to do the first step or two in a rotary tumbler if you have access to one. Otherwise, follow the directions for vibratory tumbling, with these exceptions:
Step One: Skip if your glass is not rough. If it seems advisable to complete this step, use 120/220 grit silicon carbide. You may want to use a cushioning agent -- but not plastic pellets. Wash your glass and hopper thoroughly when the step is finished.
Step Two: Use 220 grit silicon carbide and consider using a cushioning agent. Wash as above.
Step Three: Use 600 grit silicon carbide and cushion if necessary. Wash as above.
Step Four (Pre-Polish): Use 600 grit aluminum oxide as outlined in vibratory tumbling. Wash thoroughly.
Step Five (Polish - Wet): Use cerium oxide or .5-1 micron aluminum oxide; follow manufacturer's suggestions as to amount. You may use 1 to 4 cubes of sugar per lb. of load as a cushioning agent if it seems necessary. Wash thoroughly and burnish as needed.
Alternate Step Five (Polish - Dry): An alternative polishing method you can use with a vibratory tumbler is to process your glass without water. Use Vibra-Dry in three steps, using #600, then #2500, and finally #25000 in 2 to 3 day runs. You may also try simply using cerium oxide and a lot of cornmeal, running the step for 3 to 4 days.
Our 4-Step Grit Kit contains the perfect grit combination to achieve an authentic beach glass finish!
Achieving a soft, matte finish on your glass is much easier than going for a high polish, though it still may require some experimentation. A rotary tumbler is recommended, as a vibratory machine won't give you the soft curves that are part of the seaglass look. Using a cushioning agent -- plastic pellets or one of the others listed above -- will reduce chips and fractures.
Step One: Process using 60/90 grit silicon carbide if you need to remove a lot of material. Otherwise skip this step. Cushion your load. Check the progress every day, which incidentally will relieve any gas build-up in your barrel. You may not want to do this step for more than two or three days. Experimenting is the heart of creating seaglass!
Wash your glass when the step is finished.
Step Two: Use 120/220 grit silicon carbide and a cushioning agent. Check your progress daily, and stop when you've gotten the effect you want.
Instead of commercial grit, some people just use ordinary beach sand, or that sold in hardware stores for making concrete. If you want a lot of variation in your finished product, this might be for you.
Information from How to Tumble Polish Rocks into Gems by Edward E. Smith and various other sources.