If you decide to put a piece of glass (clear or opaque) over your design, be sure to add some stringers or larger frit on the edges to allow air to escape while firing. The key is to keep it very thin so the paint won’t bubble.
Glass powders can also be mixed with Liquid Stringer medium or Aloe Vera. This gel medium suspends the glass powder.
You mix the gel and glass powder to until it has a glue-like consistency. It can be put in a needle bottle or ziplock bag and squeezed out on to your glass in a design. my site