Bad finish on fired Sterling Clay

Hi, I was wondering If anyone has had any issues with the finish of sterling clay pieces after firing. My pieces go into the final stage of the firing schedule perfectly smooth and completely dry and some of them come out with a really bad, lumpy finish. I though maybe that my carbon I fire it in may be too coarse so that air may sit around the pieces and cause the lumps, so I ground it down to a finer powder and It still happens. Does anyone have any ideas as to why this is happening - I am at a loss and can't afford to keep scrapping pieces.
Many Thanks,
Charlotte

It will be a few things... the carbon is the first, I use Magic Carbon from Cool Tools or MetalClay.Ltd and its much better than the rest. Also slow down your first stage firing to a slower ramp to give the binders longer to burn out, finally I cook mine about 15 degrees lower than it suggests on the packet - your kiln may be overfiring slightly which can make the difference - still nice and strong though. Hope this helps :)