Martz Memories by Stanley Lee          Test Tiles and Glaze Stains           Tale #9


     “Get as much information from your test tile as you can”, said the master to his 1956 class students.  Then he showed us several ways to make informative tests.  Until then, some workers had sent to be fired simple glaze test samples on any old piece of bisqued pottery.  Often the test was of a single lick of glaze.  That was not good enough for Karl Martz, and he did not want it to be good enough for his students.  Hence the lesson.  We soon learned that by following his directions, we could get several pieces of test data from a thoughtful application of ingredients to a three inch ceramic cookie.  Such a test contained tactile, written, and chemical reaction information that would serve and guide us in the making of a successfully glazed ceramic piece. 


     When I started this article, nearly fifty years have gone by since I first heard those words.   After many residential moves, and changes of employment, I wondered if I could find a test tile that would illustrate my obedience to the master’s instructions.  I needed to find one that had followed me through the moves and the years.   What to do?  When all else fails, try the garage!  I did, and discovered two relics which had survived the journeys. 


     One color decoration technique Karl Martz taught us involved the use of soluble salts, or glaze stains, as we called them.  Test tile shapes became standardized.  We would roll out a 3/16 inch layer of clay onto a piece of canvas.  Then, with a cookie cutter, we cut a dozen or more tile shapes, and punched a 3/8 inch hole in the top of each to facilitate hanging from a peg board.   To show the effect of subsequent coating thickness, I scratched, with a hacksaw blade, a half inch wide strip.  To show the difference between the raw clay and a white body, the left half of each cookie received a coat of white engobe.  These blanks were dried, bisque fired and stored. 


     In their lockers, students kept supplies of bisqued blanks for all of the pottery bodies they intended to use, because a glaze could react in an unknown way on an untried body.  With a black “ceramic pencil” and on the back of the tile, I wrote more data.  This included the name of the body, the glaze reference number, and the intended firing cone number. The test cookie was ready to be glaze coated.  The top half had two coats, and the bottom half one coat.  Ten minutes later I applied brushed on stripes of different soluble salts, and I included a stripe of stannous chloride.  This tin mixture reacted with several glazes to make a pink stripe.  On some glazes it had a bleach effect.  Such a surprise could be exploited to make an eye-catching glaze on a fired piece.


     From the information I have on my S-70 F test tile, of 1956, I would know what

 to expect if I used the same formula on a cone 7 stoneware plate today.  I am

glad to see all the data is present.  The saved test-tile shows me I have been well

 taught how to preserve much knowledge on a little three-inch cookie.


    Thank you, Karl Martz!