I saw a cool idea, tried it and learned for next time. Our birds and bird baths were fired with a blue glass marble inside, the kind used in vases of flowers. Next time I will wait to add the marble during the second glaze fire, which is cooler and shorter. In the bisque firing, the beads melted and bubbled a bit too much. The children were enthralled though, at the water-looking bird bath. We kept the bowl white but painted our birds a host of water colors.