Diamond Cube Firing Failure in Porcelain: Troubleshooting & Causes

Question
Dear Mr. Lumetta,
I have been "setting" small sapphires and rubies in porcelain clay and firing them up to 2200 degrees Fahrenheit without problems.  However, I recently fired three 3mm raw diamond cubes in porcelain and they all disintegrated.  Only a trace of brownish residue was visible in the bottom of one of the "settings."  These are my questions:  1. Is it possible that I did not purchase real raw diamonds?  2. If they are real, what might have caused their complete disintegration?  3.  Any idea what temperature I could fire these and other raw diamonds (I have 2 rose cuts) to without them burning up?  4.  I would like to try firing some small (1-1.5mm)faceted diamonds, but am wondering if this might happen with them as well -- your thoughts? My gratitude. Gayle

Answer
HI GAYLE,

IT MAY BE POSSIBLE THAT YOU DID NOT GET REAL DIAMOND CUBES. THE MELTING POINT OF DIAMOND IS OVER 4,000 DEGREES CELCIUS. HOWEVER I HAVE SEEN NATURAL DIAMONDS GET 'SMOKED' UNDER A JEWELERS TORCH AT MUCH LOWER TEMPS. SOME IMPERFECTIONS IN THE SURFACES OF THE CUBES OR LOWER GRADE ROUGH MAY BURN UP UNDER THESE CONDITIONS. YOU SHOULD HAVE NO PROBLEM WITH TEMPS UP TO 3,000 DEGREES F. YOU MAY HAVE USED TOO MUCH OXYGEN IN THE MIX AND 'BURNED' THE STONES. DIAMOND IS PURE CARBON AND CAN CATCH ON FIRE IF THERE IS A GOOD OXYGEN MIX. YOU WILL LOSE SOME STONES IN ANY CASE BUT DIAMOND SHOULD BE MORE DURABLE THAN CORUNDUM AND YOU WILL HAVE TO USE SOME DISCRETION IN WHICH DIAMONDS YOU USE. ALSO CHECK WITH THE SUPPLYER AND ASK IF THEY HAVE HAD THIS PROBLEM WITH OTHER STONES OF THEIRS. THERE ARE A LOT OF COMPANIES THAT 'SET' THE DIAMONDS IN THE CASTING WHILE IT IS CAST. A DIAMOND IS A DIAMOND NO MATER WHAT THE SHAPE OR CUT, BUT FLAWS AND SURFACE ABNORMALITIES CAN CAUSE BURNING AND DISCOLORATIONS. BE CAREFUL WHICH STONES YOU USE. GOOD LUCK

PETER