How I Make Glaze Test Tiles

How I Make Glaze Test Tiles
27/02/2015 Derek

I’ve tried all manner of methods for making test tiles- thrown, extruded, and slab.  Each type has advantages, but I’ve finally come to the safest, most economical and useful method for my needs.

Assuming you have a fast way to make slabs, using this technique you can easily make a couple hundred test tiles in an afternoon.

Hand-rolled slabs work fine.  The irregular edges can be saved and used to make the supporting triangles.  To attach the supports I use dry clay trimmings and pure vinegar.  The triangles are dipped into the vinegar slip for about two seconds, then wiggled into place on the test tile until the two slabs are firmly joined.  (I have tried this with all of my clays, from porcelains to stonewares, without problems.  However, your clay may differ.)

Just after joining, the test tiles should be covered and dried slowly for the first day or so.  You may notice some cracking along the join line, but the join should be strong enough to last through bisque and glaze firings.

Edges of hand-rolled slab used for support triangles

Joining slip made with dry trimmings and vinegar

 

If you have casting slip, the fastest way that I have found to make test tiles is to make a slipcast slab using a large, flat plaster table.  Supports are joined to tiles using the same casting slip.

Cutting a slipcast porcelain slab

A fork is used for texturing the test tiles.

Type of clay is stamped onto back of each test tile

These test tiles will not fall over (especially important as I often put tests in public kilns).  They can be stacked very closely in the kiln, and the height can be varied by choosing either the long or short end of the triangular support.  Because they are flat, they require less glaze for coverage (handy when using very small batches of material) but also show more area of glaze than an extruded column.  The hole in the base allows the tile to be hung on a glaze bucket or on a display.  I stamp the back of the tile with a code for the type of clay, and there is ample space to write information about the test using an underglaze pencil.

Packed close in the kiln.

Dipping in 20ml of glaze

Right side double-dipped

Full details written with underglaze pencil and permanent marker

Taking photos with a styrofoam support

The tiles should be quite thick in order to better absorb and support thicker layers of glaze.

After dipping the face of the tile into a glaze, I dip one side (not the top) again in order to see different thicknesses.  (Double-dipping the top of the tile often results in the thicker glaze running down into the thinner area, sometimes resulting in a completely even coat after firing.)

Using flat, square tiles I can dip twice while only using 20ml of glaze.  This is especially useful when doing triaxials or biaxials using the syringe method.

The back of the tile has a lot of space for writing glaze information.  Before firing use an underglaze pencil (I use Amaco Black Pencils), after firing a permanent marker can be used to record the date and temperature.

Another advantage of these square tiles is that they photograph very well.

For my purposes, the best size for a test tile is 6cm or 7cm square.  This size is large enough to get a feel for what the glaze looks like, while small enough to be covered by only 20ml of mixed glaze (including a second dip).

Conveniently, the supporting triangles are half the width of the tile, which allows for first cutting the slab into 6cm or 7cm columns.

I know it seems really anal, but this cutting pattern saves clay and ensures the tiles all rest at the same angle, allowing for more accurate comparisons and tighter packing in the kiln.