3 min read

Mt Vision Granite: Processing

How to pulverize granite to powder.
Mt Vision Granite: Processing
Collected Granite awaiting processing

For use as a glaze material, the granitic rock needs to be reduced to a fine powder. It's not a complicated process, but it does take some time and effort.

Step 1: Gathering

Best practices dictate to always obtain permission to collect raw materials from public or private lands.

I'm lucky, I go outside in my back yard and pick it up. There is a lifetime supply.

Step 2: Calcination (Baking)

Calcination is a technical term for heating a material below its melting point for the purpose of causing thermal decomposition. In other words, we are heating the granite in a kiln so it will be easier to crush. And it works.

The granite after calcination

The granite is washed of dirt before loading it into the kiln and then fired to 1200 degrees Fahrenheit using the schedule below, which takes about 20 hours.

Ramp Rate Temp (F) Hold
1 50 180 03:00
2 50 450 02:00
3 100 1200 00:00

After calcination a reddish hue is visible due to the oxidation of the iron present in the granite. With this schedule, the larger pieces remain intact while the smallest pieces sometimes crumble. No explosions thus far.

Step 3: Crushing

This is the "fun" part. A two step process is employed to crush the granite is until it can pass through a 6mm sieve.

First: Break the calcined granite into ~2 inch or smaller pieces

For this purpose the contraption shown below was created. It's is a bag of cement poured into a wooden form with plywood siding added to capture the splatter. A short-handled sledge hammer is used to break the granite up into pieces of about 2 inches in size or less, appropriate for crushing.

Second: Crush and Sieve

The broken chunks are progressively passed 3 times through a Crazy Crusher rock crusher, after which the output will easily pass through a 6mm sieve. The Crazy Crusher is very efficient and is much easier and faster than when this whole process was performed as shown above using the sledge hammer (phew!).

The helpful Crazy Crusher.

The granite after sieving looks like this.

Step 4: Milling

Next the crushed granite is milled into powder using a ball mill and a 5-liter porcelain vessel filled with porcelain balls. The vessel is mechanically rolled on its side to create a tumbling action.

Ball Mill, porcelain balls and porcelain cylinders. The cylinders are spun on their sides creating a tumbling action for the balls to pulverize the granite.

The granite is milled "wet" using a mixture of 2.0 liters of granite and 2.5 liters of water per 5-liter batch. Milling takes 12 hours (overnight).

Step 5: Sieving and Drying

Once milled, the wet batch is sieved into a bucket using a 60M mesh and then left to stand overnight. After pouring off excess water the charge is transferred into a plaster-of-paris elliptical platter for drying (below).

Plaster-of-paris elliptical platter used for drying the milled granite

Some days later the powder is dry and can be stored for use. And we are done!

Final Product - Mt. Vision Powder

Notes:

  • Two liters of crushed granite results in about 6 pounds of glaze material post milling, sieving and drying.

References

  • Hamish Jackson posted an entertaining video that describes his technique. You can find it here on Youtube. Later Hamish discovered the Crazy Crusher. My process, with small variations, is modeled after his.