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 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.

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 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.

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).

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

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.

Milling Tests

Over time it became apparent that rock selection, milling and batch size can have a big impact on the final product. To begin to understand this better, the following tests were performed.

Five milling tests were conducted. Each test used granite rock from the same calcine batch, which was sorted to consist entirely of Granodiorite (so as little Granite as possible).

The granite was crushed until it passed through either a 6 mm or 3 mm seive. It was then loaded and milled in a 5-liter porcelain jar using porcelain balls as described in the table below.

A glaze from each sample was formulated as follows: 150 gram batches of 70% granite and 30% Wollastonite, mixed with 150 grams of water that produced a glaze with a specific gravity of ~1.5-1.55 (so a bit thick).