You can mix metal clay with some man made and natural stones. BUT NOT ALL. Many stones cannot be fired or there are limits on the time and length of firing. So today I am going to talk a bit about this issue of firing stones in metal clay.
First, a little bit about naturally occurring stones. One misconception is if the stone is considered hard, then it be will OK. Well this is wrong. What do I mean? One way of measuring stones is using a scale called the Mohs hardness scale. It was developed by Friedrich Mohs a mineralogist and is one of the ways to define the hardness of a mineral. Its principle is based on the scratch resistance of a mineral and is defined but the ability of one mineral to be scratched by a mineral higher on the scale. The scale is number 1-10. One is Talc and ten is Diamond. You can read more about this at All About Gems. Sadly, the Mohs hardness scale does not really help in determining the success of firing gemstone in metal clay.
Here is an example:
- Hematite which is 5.5-6.5 Mohs hardness is deemed fireable at 1650 for 2 hours.
- Diamond which is 10 Mohs hardness is not fireable.
- Green Tourmaline which is 7-7.5 Mohs is deemed fireable at 1220F for only 30 minutes
One of the problems with natural stones is that there may be inclusion in them and these little inclusions can be tricky, the heat may create stress and cause the stone to crack during firing.
Another little thing about stones is colour. Many stones will change colour with heat. Heat is actually used to enhance colours of natural stone and to change the colour (but that is another story). Some stones will become brittle and crumbly due to the heat of firing. I have fired beach rocks to see what would happen. The result have been varied some will be fine, some will become brittle and some will change colour.
The best way to find out if something will fire safely is to check a gemstone firing guide there are several published the most comprehensive one I can find on the web is one from the Cool Tools website Cool Tools Gemstone Guide. This resource has great information about firing gemstones in general and their firing guide. This post only touches the surface so if you want some more information to devour this is a great source. When looking at firing guides, I prefer the ones that highlight both maximum temperature and length of firing.
TIME and TEMPERATURE need to be considered when you are firing gemstones. Both these variables can play a role in successful firing. A temperature that is too high will affect the stone and a long firing may also affect the stone.
Here are two examples:
- TEMPERATURE: Labradorite is deemed a low fire stone fireable at 1200F for 30 minutes. I had some inexpensive Labradorite beads and decided to fire them at 1600F just to see what happened. The changed completely and turned coral and orange colour.
- TIME: Before I had found a firing guide included time I fired a lab created emerald which is deemed safe at 1470 for 30 minutes at 1470 for 1 hour. It changed from emerald green to brownish garnet. The increase in time for another 30 minutes caused the stones to change colour.
Lesson here is the be cautious and find a good reference.
Sometimes test firing your stones before you set them in metal clay is a good idea. Take your stones wrap them in a fire blanket and kiln fire them exactly how you plan on firing your piece and see what happens.
If in DOUBT leave it OUT!!!! If you have some stone that you absolutely love and don't want it to be destroyed set it after firing your metal clay. Heat can do crazy things to materials.
One last thing, I learned something new today. In the reference from Cool Tools there is a section on carbon firing. Apparently one reason gemstones change colour is due to heat in the presence of oxygen. They proposed that if oxygen was removed would a gemstone survive higher heat??? Carbon is a medium used when firing copper and bronze clays has the property of absorbing oxygen. Cool tools has tested this theory firing gemstones in carbon to see if some stones will survive high heat if fired in carbon. The answer is they had success for many....see the Cool Tools Gemstone Guide for details.
I hope this helps.