Many natural gemstones can be set into metal clay and fired in place. Other Red Agate gemstones will not survive the heat of a kiln and should be set after firing. The charts below show the results of kiln and torch tests that have been performed on both natural and synthetic gemstones, adapted with permission from the original testing by Kevin Whitmore of Rio Grande, with additional testing by Cool Tools.

This information is for reference and should be used as a guide. There is always some risk of losing a natural agate jewelry gemstone even if others of it’s kind have survived in the past. Gemstones may have internal flaws that can be liquid or gaseous filled, or contain crystals of other materials that can cause the gemstone to fail where it usually does not. This guide aims to help metal clay artists sort out gemstones that are known to survive under fire from those that are not.

Gemstones are minerals that are classified into groups based upon the constancy of their major properties. Each mineral family has one or more varieties contained within the group. When we sort the tested Red Agate gemstones according to their mineral group, it becomes clear that an easy way to gauge the survivability of a gemstone is to look at the results of other varieties within that same group. Aquamarine and emerald, for example, are both varieties of the beryl group of minerals. The result of tests done on aquamarine and emerald indicate that minerals in the beryl group will not survive kiln heating. There are exceptions, as there always are in the natural world, but in general this method can be reliable for many varieties. For simplicity, many gems have been classified into their main group rather than their sub-variety.

These charts classify stones as No-Fire, Low-Fire, High-Fire, Torchable and Carbon-Safe. High-Fire stones are those that can withstand 1650F for at least 1 hour on an open kiln shelf with no color change. Low-Fire stones are heat sensitive with a risk of color change. No-Fire stones are those that must be set after firing. Torchable stones are those that can survive at least a 2 minute torch firing, and Carbon-Safe stones are those that can survive a carbon firing (carbon firing instructions are included at the end of this publication).

The Moh’s hardness of each gemstone has been included to help dispell the myth that hardness determines survivability (additional information on the Moh’s scale is provided on page 3 of this publication).

The Moh’s Scale

The Moh’s Scale is a non-linear scale of hardness defined as resistance to scratching or abrasion.

On the Moh’s scale, 10 represents the hardest material and 1 the softest. A diamond can scratch another diamond and anything softer than itself. A corundum can not scratch a diamond, but it can scratch another corundum and anything softer than itself, and onward down the scale to talc.

In viewing the Moh’s scale, it would appear that corundum is almost as hard as a diamond, but the Moh’s scale is deceptive because it is non-linear. It has steps from 1 to 10, but they are not equal steps. If we compare the same chart of minerals on an absolute hardness scale it is apparent that rubies and sapphires (corundum minerals) are actually 1/4 the hardness of diamond; not 90% as the Moh’s scale would seem to suggest.

When looking at the maximum recommended firing time and temperature for a given agate jewelry gemstone, it can be disappointing to discover that some stones are so sensitive that they can only stand a few minutes at a very low temperature. That means that some stones can only be embedded in low-fire clays, and if you wanted a very strong product or wanted to use a clay that required a 2-hour firing, you’d have to select a different gem.

The kiln and torch firing tests conducted on natural, lab and synthetic Red Agate gemstones were all originally conducted before the introduction of bronze and copper clays. The gems were tested either by firing them alone in the kiln on an open shelf, firing the stone alone by torch, or firing a stone embedded in silver clay. So, in every case, oxygen was present during firing.

The new copper-based bronze and copper clays are buried in activated carbon during firing to avoid the creation of fire-scale that is difficult to remove from the surface of the fired metal. Fire scale is created when copper is heated in the presence of oxygen.

Carbon is used as a firing media for copper-based clays because one of its properties is the ability to absorb oxygen. Since oxygen is the element that causes fire-scale to form when copper alloys are heated, it follows the same theory that it should also protect gems that are sensitive to oxygen.

Firing a Carbon-Safe Stone in Bronze or Copper Clay

Firing agate jewelry gemstones in copper-bearing clays is very simple. Embed your chosen stone in your bronze or copper clay, dry and fire as usual in activated carbon.

The carbon adsorbs oxygen during firing, and the lack of oxygen protects the stone from burning. Any stone marked as Carbon-Safe is oxygen-sensitive, but can be safely fired when protected by activated carbon.

Firing a Carbon-Safe Stone in Silver Clays

With this technique, you can safely fire stones in all forms of silver clay at the ideal temperature without sacrificing the stone or the integrity of the final product.

The method is simple. Pre-burn the binder from the clay before burying it in the activated carbon. The binder must be burned away first because it will not combust in activated carbon with silver clay present.

To pre-burn the binder, place it on a bed of vermiculite to allow air circulation around the entire piece and ignite it with a flame. Allow the flame to burn itself out, then turn it over and repeat on the back side to be sure no binder remains. When igniting the clay, use a flame with no oxygen added to it (such as a standard lighter). A cool flame is needed so the stones are not overheated during burnout.

After burnout, bury the item in activated carbon and fire at any temperature up to 1650F for your desired hold time.

For a PMC Standard firing, which requires a 2 hour hold time, set your hold time to 2 hours and 30 minutes if you are using the standard bronze or copper clay firing vessel. The time must be extended as the carbon delays the heat reaching the interior of the container by 30 minutes.

For PMC PRO firing, no pre-burn is needed. Bury the item in the activated carbon and fire at 1450F for 1 hour.