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Published Jun 19, 2023

When testing ceramic glazes, it is extremely important to take good notes so that you can learn from your results. I am embarrassed to admit how many times I put a test tile in the kiln, positive that I would remember what was on it. And guess what? After the firing, I had no idea what I did to create the results.

In today's post, an excerpt from the June/July/August 2023 issue of Ceramics Monthly, Rhonda Willers shares some great tips for getting the most out of ceramic glaze testing. –Jennifer Poellot Harnetty, editor

PS. To learn how Rhonda uses spreadsheets to track her glaze testing in the studio, check out the rest of this article in the latest issue of Ceramics Monthly!

Methodical Organization

To maintain an organized testing practice in your studio and to avoid costly errors, consider the following preparations:

  • Make test tiles in advance. Produce multiple test tiles or test objects (up to 100 or more, if you’re really adventurous) that are ready to use when you are ready to start applying and firing terra sigillatas.

  • Dedicate a studio notebook/sketchbook to surface notations and record keeping.
  • List out what you wish to test before beginning the process.
  • Create a labeling system for test tiles and containers that makes sense for you. Record both labeling systems in the front of your notebook for easy reference.

Note Taking and Record Keeping

In the process of trying new material ideas, there is nothing more frustrating than having a wonderfully exciting result and not knowing what you did or not knowing what someone else did to achieve it. In my own practice, when trying a new layering or application process, I find that I need to write down what I plan to do first to ensure that I follow through with the record-keeping part, especially when I’m working on applying multiple layers of terra sigillatas and washes. This takes the form of: A quick sketch of the piece or written description, a list of materials I have out to use, and adding numbers to the materials once I know the order that I applied them. These process numbers are added after I have applied the terra sigillata and wash materials to the form. 

1 Sample record keeping of the application process. 2 Application notes photographed with results for record keeping purposes.

When making multiple surface, glaze, or terra-sigillata tests, I’m more regimented about the record-keeping process. I make a spreadsheet list (or record in my dedicated studio notebook) of the tests I will be completing. Within the spreadsheet, I create a labeling system with numbers and letters if there is too much information to record on the back or bottom of a tile. 

I prepare corresponding labels for containers and attach the labels before mixing materials. In my sketchbook, I also detail the application method (1), final kiln temperature, cones used, and any other relevant information about the process that could be considered possible impacts or influences on the results. After the tests are fired, I record the results in the spreadsheet or in my sketchbook (2). If I am feeling particularly thorough, I take photographs of the results to include in the final records.

No matter what method of record keeping you prefer, the main goal is to have enough information to be able to repeat, or avoid, the results you discovered through your testing exploration. 

Digitally Recording Results

Using Microsoft Excel, or a similar spreadsheet program, you can create a workbook of multiple spreadsheets to plan your line blends or other terra-sigillata testing. The screenshot below (3) shows line-blend planning using an OM 4 ball-clay base terra sigillata (gray box) and a Grolleg kaolin base terra sigillata (light blue box).

3 Using Microsoft Excel to create charts for planning terra-sigillata testing.

In the next row are the standard line blend measurements from 1⁄8 teaspoon to 1 tablespoon. These measurements are repeated under each base terra-sigillata heading. The next rows are the colorants that you want to test. Below that you will see there are additional colorants listed. These were separated to represent a secondary set of testing. The first groupings were the priority tests. 

Inserted between the colorant lists are a few summary lines that indicate where these color line blends would be tested and allow for space to record how many test tiles needed to be made to accommodate the intended testing. Next, the three firing methods are listed. 

In the upper left of the spreadsheet, a notation of 36 tiles/colorants is noted. This represents testing a colorant, such as cobalt carbonate, in four base terra sigillatas (only a portion of this is shown in the screenshot).

Excerpted and adapted from Terra Sigillata: Contemporary Techniques by Rhonda Willers. This book is available in the Ceramic Arts Network Shop at