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Gas In the Rock Tumbler Barrel?

Glass in a tumbler barrel
Some types of man-made and natural glasses can generate gas when tumbled. Keep an eye on barrel pressure when tumbling these materials or you might be cleaning up a terrible mess!

It's True, Glass Makes Gas!

One day I started my Lot-o-Tumbler with a load of landscape glass and it was running great. When I checked the tumbler the next morning the lid had popped off and the slurry had evaporated to a thick mud. I didn't think too much about it. I assumed that I didn't have the lid firmly seated. So, I rinsed the glass and started the tumbler again.

Later that day the lid was on the floor again. I was really surprised because I had intentionally placed the lid on firmly. It happened a third time and that convinced me that the glass was producing enough gas to pop off the lid.

This problem was easily solved. I simply poked a tiny hole in the lid of the Lot-o-Tumbler and that allowed future gas to escape.

A build-up of gas in a rotary tumbler can be a more difficult problem. If the lid pops off the stones and tumbler mud will go all over your tumbler and all over the floor. We have cleaned up this type of mess. It is not a fun mess to clean-up.
Rough and tumbled snowflake obsidian
Natural glass such as obsidian and Apache tears can produce gas when tumbled.

What Materials Produce Gas?

Most of the rocks that people run in a tumbler do not make enough gas to cause a problem. However, we have encountered several materials that are likely to produce gas. We list them below to warn you.
  • man-made glass (especially colored glass)
  • volcanic glass (obsidian, apache tears)
  • metallic ore minerals
  • metals
  • blast furnace slag
The amount of time that a material spends in the barrel can also be important. Some people (including me) will often add a little extra grit and a little extra water and tumble a barrel of rocks for two or three weeks without opening it. In that situation a rock that produces very little gas can have the extra time needed to build up enough pressure to unseat a lid.
Gas build up in a tumbler barrel
Check your tumbler barrel often if you are tumbling glass. In this image you can see that gas pressure is causing the bottom of the barrel to bulge out.

Watching for Gas Build-Up

If you are running a rotary tumbler with a rubber barrel it is easy to watch for a build-up of gas in the barrel.

You should check on your tumbler a couple times per day. When you do this you should be able to detect if the barrel is inflating.

The next time you start tumbling, examine the barrel - especially the bottom. Look at the shape and press on the bottom with your finger to become familiar with its shape and tension. Then when your are checking on the tumbler pick up the barrel and look to see if the bottom is bulging or if it is harder to press in with your finger.

A bulging barrel or a hard barrel is a sign of gas pressure in the barrel. A small amount of rock mud seeping around the edges of the lid can also be caused by gas inside.

Relieving Gas Pressure

If you detect pressure inside, place the barrel in a plastic tub or in your laundry tub, or take it outside. Wearing eye protection is strongly advised - you don't want grit in your eye! Then, slowly and gently start opening the lid, with the opening area pointed away from you. It is unlikely that anything will spray from the barrel but take this precaution just in case. Some people call this "burping the barrel". Some rock gas has an odor of sulfur or rotten eggs or a struck match.

Before restarting the tumbler make sure that the rim and the lid are clean and no grit or rock particles are going to compromise the seal between them.

Thumler's plastic lid with breather hole
Shown here is a Thumler's 2-pound and 3-pound barrel lid with a small hole in the center. Cover the hole with tape during tumbling and use the hole to relieve any pressure that builds up.

Our Method For Relieving Gas On Thumler's Tumblers With Plastic Lids

To use the following method we recommend purchasing a extra lid that will only be used when tumbling gas-producing materials.

We did not like burping the barrel as described above so we found an easier and cleaner way to relieve the pressure. We drilled a very tiny hole in the center of the lid and covered the hole with a small piece of tape during tumbling. To relieve the pressure we take the barrel off of the tumbler, and place it on a table with the lid facing up. We then gently peel back the tape.

If there is a build up of gas in the barrel you might notice a hissing, bubbling, or an odor. We then clean the lid of the barrel, dry it thoroughly, and apply a new piece of tape. We then place the barrel back on the machine and continue tumbling.

If you are tumbling a gas-producing rock, checking the tumbler frequently is important. Check no fewer than two or three times per day.

Reducing Gas With Baking Soda

Some people say that they add a teaspoon of baking soda to their tumbler barrel to counteract gas. This probably works with some materials. However, we have not had 100% success with this method -- even when using a tablespoon of baking soda. So, instead of taking chances we use the methods described above.

Happy Tumbling! Authors

Hobart King Hobart M. King: Most of the articles on this website have been written by Hobart King. He is owner and manager of and has decades of rock tumbling experience. He has a PhD in geology and is a GIA graduate gemologist. He has also written most of the information about rocks, minerals and gems on the website.