Making test bars is a great way to test out a new clay and discover how it will fire in your kiln. I’ve made a lot of quick and ugly test bars that get discarded after I’ve recorded the final look and colour, shrinkage, warping and maturity of the new clay. In an effort to not waste anything I’ve recently started making test bars that double as herb labels.
New clays for testing
Roll out each new clay for testing and allow the clay to firm up a little so that it is easy to shape. Cut out a bar with a spike at one end. Scratch a line across the clay and measure 10 cm from the line and scratch another line. This is the 10 cm gauge for measuring how much the clay will shrink after firing. Find a way of labelling the clay type on the bar. I stamp a number on the test bar and write the corresponding clay in my notes.
Adding two lines 10cm apart and a number for each clay type
Stamp or carve herb names onto one or both sides of each stick being careful not to cover the 10cm lines. Make more than one test bar for each clay if you want to test the results in different firings.
Two test bars for the same clay: one for a cone 6 firing and the other for cone 4
Bisque fire the test bars and rub a contrasting colour stain or oxide into the letters. Sponge back the excess pigment. The bars are ready for glaze firing.
After firing, measure the distance between the two 10 cm lines to work out the shrinkage rate of the clay. The bars can be used for an water absorption test to see if the clay is vitrified. Test bars are also a great way to find out what the fired colour will be, how smooth the fired surface grain is and whether there are any surprises like bloating or warping.