Are you new to rock tumbling and have questions about grit? Maybe you are asking yourself if you are using too much or too little grit for tumbling. What kind of grit should you use and can you make your own?
For most hobbyists, we recommend the Polly Plastics Grit Kit which gives you all four types of grit for each stage of the tumbling cycle.
This article covers everything you need to know about grit.
In this article:
- What is rock tumbler grit?
- Silicone carbide grit
- Rock polishing grit
- How much grit to use
- Can I reuse grit?
- What grit sizes and polish do I need?
- What grit do I use for vibratory tumblers?
- Typical grit sizes for tumbling
- What grit should I use with coins?
- What to look for when buying grit
- Can I make my own grit?
- How can I safely dispose of used grit (or slurry)?
- Where to buy
What is rock tumbler grit?
Grit is the media used to grind down and shape stones or other objects in a tumbler and is made of silicon carbide.
This is not to be confused with grit used in lapidary machinery like saws and grinders.
Silicone carbide grit
It is important that silicon carbide grit is used as rock tumbler grit because it has a hardness of 9 on the Mohs scale and doesn’t round out when being tumbled.
Because grit is so hard, as you may have noticed in your tumbling, some softer materials containing different mineralization elements such as calcite may be gouged out and can give the rocks a texture.
One of the more common stones where you will see this type of gouging is Jasper.
Because rocks vary in hardness, it is standard practice, for pre-polish and polish stages, to separate the rocks out by hardness.
The reason for this is that rocks that are harder will grind rocks of lower hardness, preventing a high polish.
Rock polishing grit
When people talk about different compounds to use for tumbling, they are referring to the polishing grit.
This is a form of grit that is so fine that it gives the stones a polish. Thus we call it polish.
The most popular types of polishing compound are:
- Aluminum oxide
- Tin oxide
- Chrome oxide
- Cerium oxide
Aluminum oxide is the typical polish most people use and is the cheapest.
Aluminum oxide, tin oxide, and cerium oxide polishes are good all-around for polish.
Tin oxide produces better results for softer stones, and cerium oxide provides better results for glass and obsidian.
Chrome oxide is mainly used for harder stones most commonly jade and nephrite.
It is important to note that you should not mix polishes; doing so may hinder the results that a polish would achieve on its own.
How much grit to use
This is a question that many people ask when they are starting out.
I tell everyone that only one tablespoon of grit per four pounds of rock is needed.
A rule of thumb is that, if you see grit at the bottom of your barrel after one week, you are using too much grit.
When it comes to polishing, I find that I can add one tablespoon of polish per pound of rock and sometimes even more.
Adding enough polish compounds to thicken to a slurry helps to cushion the stones from breaking.
It is also recommended to use plastic pellets to help cushion the stones as the polish stage is where you will notice the most breakage.
Can I reuse grit?
Because grit gradually breaks down as you’re tumbling, you cannot reuse it.
However, the slurry that your tumbler creates can be used from the previous stage to help the grinding action.
For the polish stage, you do not want any slurry or grit in the barrel from your prior stage.
I recommend that you thoroughly wash your rocks before putting them in polish – you also need to wash the barrel.
Failing to do so will hinder your final polish results.
Unlike grit, polish can be reused several times, but eventually, it will need to be changed.
What grit sizes and polish do I need?
The standard three grits, 60-90, 150-220, and 500, and polish are what you will need for any tumbling.
Sometimes people use 45-70 grit for faster grinding on hard rocks.
What grit do I use for vibratory tumblers?
When using vibratory tumblers, it is best to use the grit that is a strait grade as 60-90 grit contains sizes ranging from 60 to 90 meshes.
If you use a vibratory tumbler, be forewarned that the lower size grit you use, the faster you will need to replace the bowl.
I like to do low number grit tumbling in a rotary tumbler. After low grits are done, I would use pre-polish grit and polish in a vibratory tumbler.
Typical grit sizes for tumbling
You will typically start out with stage 1 using either 45-70 or 60-90 grit.
This stage is usually run for one week. After a full week, some rocks will need to be put back into a new batch of stage 1 to get the desired round (or pitless) look.
It is hard to say how long stage one should take because everyone has their preferences.
Depending on if you want a more natural look or a flawless tumble, stage 1 can take as little as one week up to many months.
Stage 1 is where you decide what shape you think is good enough.
Stage 2 (using 150-220 grit), is designed to smooth out small rough edges and remove the larger scratches along with any pitting.
This stage typically takes one week as well.
Stage 3 (using 500 grit), prepares the stone for the final polish, sometimes giving it a polish look.
Stage 3 can take as little as three days to 1 week.
Stage 4 is where you use your polish which takes one week to 1 month depending on the desired polish and material type.
What grit should I use with coins?
Some people like to tumble modern coins found with a metal detector.
The best grit to use on coins is pea gravel.
Before you tumble your coins, you will want to separate your valuable old coins from your clad as there are less destructive cleaning methods available to clean valuable coins.
Also, you will want to separate your copper from your other coins. If you have pennies with your nickels, dimes, and quarters, you will turn them to a copper color.
Pennies struck before 1983 are copper, so be sure to pull those out (if you can even read the dates).
If you tumble zinc pennies, be aware that some may not survive tumbling due to the nature of how zinc oxidizes.
Beyond coins, if you want to shine up other metals, crushed walnuts are the media of choice.
Make sure that you dry off any metal immediately and put it in an area that gets much airflow. Leaving metal wet after tumbling will cause it to tarnish and oxidize quickly.
What to look for when buying grit
The biggest thing to look for when purchasing grit is a trustworthy source. For quality grit, you can expect to pay around $5 per pound.
Cheap grit tends to have lots of bubbles and contaminants which usually looks like popcorn under a microscope.
Cheap grit is also a little bit softer than better grit which means it will not last as long.
Good grit will usually look like crystal shards or broken glass with sharp points.
The cheap popcorn grit is less efficient, and you will need to use much more compared to higher quality grit.
Can I make my own grit?
Unless you work at a chemical factory or a place that you can obtain silicon carbide or other polish compounds mentioned earlier, you cannot make homemade rock tumbler grit.
It is a common myth that you can use sand or river silt to act as grit substitute. These materials round out when tumbling and become ineffective very fast.
Also, these DIY materials are usually not as hard as silicon carbide, so it will take a lot longer to achieve results.
Garnets, for example, may be as hard as grit, however, they still round out when being used as grit.
You can, however, add a small amount of sand or silt during the beginning of your grinding stages if you have no used slurry.
This can help speed up the grinding action until the rocks create their own slurry.
How can I safely dispose of used grit (or slurry)?
Grit and therefore slurry are nontoxic.
The slurry will be the fine sediment left over in your tumbler. Do not dump slurry or clean your rocks in a sink or bathtub.
When the slurry hardens, it acts as cement and is very hard to break up.
Unless you are tumbling heavy loads of hazardous rocks, disposing of the slurry in your woods or driveway is fine.
Sometimes, dumping slurry on grass can make a dead spot because it suffocates the grass roots from light and water.
On my gravel driveway, I like to mix the slurry with gravel to fill in potholes.
If you live in an area where you do not have a yard, dump the slurry in a container and let it dry out. You can then dispose of it in your trash can.
Where can I buy grit?
You can buy grit as your local rock shop, but it can be much more convenient to buy on websites like Amazon.
Some grit manufacturers sell in bulk, such as Lortone.
Most commonly grit is sold in 1 and 5-pound containers.
By fully understanding grit and how it works, you can achieve better tumbling results at a fraction of the cost.