What effect did your solid marbles give, and what cone did you fire at? When I had kids use solid marbles the marbles melted, but they had air holes and were kind of bumpy fired at 05. I was pleased with the clear marbles, but may not have fired the solid marbles hot enough???? Just guessing.
Also, the kids discovered that putting a mixture of the clear (flat) marbles with the colored ones gave a spectacular effect! The solid red ones turn a gorgeous yellow-orange and swirl with clear when they melt. The flat marbles- used in flower arrangements come in turquiose, irredescent, and a variety of colors.