A bisque tile is a small slab of your own clay, fired once, kept ready for glaze tests. A box of them is the most useful thing in a studio for anyone testing glazes, because every question about a glaze is answered by firing it on the body you actually use.
Making them properly
Roll a slab of the body you use, cut tiles, and give each one a vertical face by bending the top over or standing it on a foot. The vertical part is essential: it shows whether a glaze runs, which a flat tile never does. Punch a hole for hanging, bisque them all at once, and keep a box ready.
What to record
Write the glaze, the number of coats and the date on the back with an underglaze pencil before firing, not after. A drawer of unlabelled beautiful tiles is a common and completely avoidable outcome. Recording the body too matters if you use more than one, since the same glaze on a different clay is a different result.
Testing thickness, not just glaze
Dip a tile once at the bottom, twice in the middle and three times at the top, and one tile then shows the whole thickness range. On transparent, reactive and celadon glazes that variation matters more than the recipe, and it is the single most useful thing a test tile shows.
Buying them ready made
Suppliers sell plain bisque tiles for glaze testing and for decorating, which saves making and firing your own and costs little. They are made from a standard body rather than yours, so results transfer only approximately, which for a first look at an unfamiliar glaze is usually enough.
Questions people ask about bisque tiles
Why do test tiles need a vertical face?
Because a flat tile cannot show whether a glaze runs, which is the thing most likely to ruin a kiln shelf.
What should I write on a test tile?
The glaze, the number of coats, the body and the date, on the back with an underglaze pencil before it is fired.
Can I buy bisque tiles?
Yes, suppliers sell them cheaply. They are a standard body rather than yours, so results transfer approximately.