Nothing can be more frustrating than to find that perfect glaze test, but not be able to reproduce it consistently in the kiln. I am not talking about those basic glazes that are created from a single recipe. For the most part, anyone can mix up a glaze recipe and apply it get similar results. But what about those subtle effects, built up with multiple layers? Some days feel like I am chasing a unicorn, or depending on the day, a Narwhal. So once you have that perfect glaze test in hand, then what?
Trial by Fire
Review your notes. Like most people, I recorded only the most basic information whenever I was testing a new glaze. If I was lucky, I wrote down the name of the glaze somewhere on a piece of paper, hoping I could find it later. Taking good notes never made my Top 10 list of reasons I love pottery. But over the years, I found a few extra details went a long way in being able to reproduce results months or even years later. Currently I record:
- Name of the Glaze or Glazes
- Application Method
- Firing Schedule
- Specific Density
I talked about Application Methods in Finding a Glaze… Part 1. But let me say a little more about those last two bullet points.
Most people talk about what atmosphere and temperature they fire their pottery kilns to. For example, I work in mid-fire ceramics and fire my electric kiln to Cone 6 or about 2232°F. But that’s only part of the story. It is also important to note when you hold temperatures to let the heat “soak” into the pottery. My current firing schedule looks something like this.
- Rate 100°F, Target 220°F
- Rate 450°F, Target 1500°F
- Rate 450°F, Target 2100°F
- Rate 100°F, Target 2150°F, HOLD 1 hour
- Free Fall, Target 1700°F
- Rate 50°F, Target 1600°F, HOLD 1 hour
- Rate 50°F, Target 1500°F
It takes about 16 hours to complete this cycle, not including the final cool down which is another 24 hours. Besides the wonderful information provided by John Hesselberth and Ron Roy, in their book Mastering Cone 6 Glazes, you can also find firing schedules shared by other pottery masters. Steven Hill who works magic with glaze techniques in an electric kiln.
The key point… nothing is forever. I started firing my kilns using the standard Skutt programs. But over the years I have shifted between many schedules including patterns based on John and Roy’s work, and am now exploring Steven’s sample schedule. Be sure to record you firing schedule and give it a specific code which can be written on your glaze test.
Another key point often overlooked, is the amount of water added to your glazes. Again, this does not make much difference when working with basic glazes. But if you are trying to reproduce complex patterns it is important to know not only how many layers have been sprayed, but also how think they glaze should be. Like other potters, I had my own techniques for getting my glazes close enough. Seeing how well the glaze attaches to a tool or your fingernail works most the time. But if you really need consist results, nothing works better than measuring the specific density or relative density of your glaze. The easiest way I have found to do this is:
- Mix your glaze just the way you like it.
- Using a “special” container, weigh a specific amount of your glaze.
It’s that easy. Now anytime you need to mix up a batch of your glaze, big or small, just use that same “special” container to weigh your glaze. Add enough water until it matches your desired weight and you will always have (approximately) the same specific density without worrying too much.
Here are a few samples from my last kiln firing (Test 1 and Test 2). I started by finding interesting glaze tests on the walls of my studio. Often it is only a small piece of a sample that catches my eye, but it makes me want to see more. So I check my notebook for details about the glazes, including specific density, application techniques, and kiln schedule. Put that all together and I am able to recreate the glaze to my satisfaction.