Les,
Welcome to IAP. While you are here check out the Ohio forum. Some of us in the area get together every couple months, usually at Hartville Hardware. The next meeting looks like it's going to be in February.
I've never tried coal, unless that is an auto correct from Gold.
Have you done successful casts before, and is this a new issue since trying to do the new pigments?
Have you tried just a clear cast with no additives/pigments to see how its comes out? 1-2 sample runs of just clear castings is worth the time spent to know that you have it right before adding more variables.
Are you using Polyester Resin, if so how many drops of catalyst are you using?
We need a bit more information to help get to the bottom of the issue.
Mike.