Mike -

I agree with Mark; sounds like you are getting the material much too hot! I have used the following process to polish lapis from NYS, Colorado, California, Chile and Afghanistan. I work lapis primarily with silicon carbide sanding belts on 8" diameter expandable drums.

I use a fresh 220 belt for shaping, 400 for sanding, and worn 600 for the final surface preparation for polishing. All three sanding stages are done very wet. For final polishing I used an 8" rotary flat lap (running approx. 450rpm), with a hard leather surface. The leather has chrome oxide well worked into the surface. I put a thin coat of chrome oxide slurry on each cab and allow it to dry completely (a 100w incandescent bulb speeds the process). I then mist the leather lap with water that has a little vinegar in it for acidity and start the machine. I press the cab gently but firmly into the lap, beginning with the very top of the cab. When the cab begins to "grab" the lap I tilt it, moving down and around the cab, finishing with the "girdle. Rarely does one sequence result in a high polish. Equally rarely are more than three sequences needed to yield an almost optical polish. If an orange peel or smeary surface appears at any point, it signifies overheating and necessitates returning to fine sanding and pre-polishing.

KOR,

Jim Small
Small Wonders Lapidary
Church Hill, TN

From jim small - October 27, 2009 at 10:43:19
Email: jsmall47[ ]embarqmail.com

Message: 68471