Martz Memories, by
Terra Sigillata and Sealing
From a dictionary study of word derivations we learn that a sigillate is intended to seal. The ancient Greeks used the word because they had a sealing need. Much of the earthenware pottery and vases that came from their kilns was covered with a black pine-tar like coating, but they carved through this layer to expose the clay beneath, and left us their beautiful red and black designs. But there was a problem. Unless the red clay designs could be protected from food and drink stains, or from frequent handling, the art work would be obscured. Some kind of durable over-coating was needed to “seal” the finished work.
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As part of his teaching, he had urged us to standardize our glaze making and glaze mixing practices. This enabled us to localize problems if they occurred. Batch glazes were mixed with the aid of a ball mill. The mill was comprised of high-fired ceramic containers that held about a gallon. These jars were loaded with glaze ingredients, water, and some pebbles to assist with the mixing process. The ball mill jars were capped and placed horizontally on two rollers which were electrically driven.
Mixing time was standardized to about a half-hour, and not altered unless there was a special reason to vary the time. Karl gave us the reason for standardization. He explained that a prolonged mix would start to grind the ingredients more than was necessary, and that these finer particles could produce effects in the fired glaze that were different from those we would normally expect. We noted his words of caution and kept our mixing times to half an hour.
Imagine our surprise when one day, we saw him put the same jar with its glaze contents on the mill rollers and leave it there for several hours without even checking to see how well the contents were mixed. A class-mate jokingly suggested the ingredients would look like soup when the jar was opened. They did!
Eventually, Karl took the jar from the rollers, opened it, strained the contents to remove the pebbles, and poured the “soup” into a gallon glass jar. The jar was put on his shelf, and he left the room, because he was not ready to do any explaining. The next day at class-talk time, he produced the glass jar. The contents had separated with the soupy part in the bottom third, and almost clear water above it. Now our teacher was ready to talk about his unusual behavior which was in contrast to earlier teaching.
“We don’t need the water, so we’ll siphon it off. We want to use the finely ground clay slurry which is what the Greeks called ‘Terra Sigillata.’ In use, it is poured or gently brushed over all the outside of the carved pot or vase. Then the piece is fired again. The finely ground clay acts as a thin glaze, and provides a gloss seal which protects yet reveals the finest details.” To me it seemed a lengthy process, but we were not early Greeks. However, it worked for them.
The term “Terra Sigillata” rested in the back of my memory, and it remained there for many years. My employment had me teaching grade-school teachers how to use different art forms to help them with their lesson-teaching. One method used clay, and involved an easy way to sculpture a life-size head. The teachers enjoyed their work, and we bisque-fired their sculptures.
Several went on to create another head, and I sculpted with them to show them some abstract impressionism ideas. Soon there were more than twenty bisque-fired heads sitting on top shelves, although the semester was coming to an end. We had not considered ceramic glazing, since that process went beyond the scope for most grade school teachers, but I realized the heads needed protection of some sort. Then I recalled the early Greeks’ problem. Perhaps the Greeks did have a word for it, but grade-school teachers could not be expected to grind “Terra Sigillata” to seal the sculptures they had made and bisque-fired. Was that extra firing necessary? Their work would pick up dirt and perhaps grease with time, and so would my own sculptured heads which by this time had become heads and shoulders. As it was, there were still more student heads to be fired.
Late Friday
evening found me firing the kiln with the last of the heads nearly
bisqued. About
“But won’t the wax wear off with all the foot traffic?” I asked.
“Eventually, but it dries hard, and wears well. Once a month we scrub the floors and wax to seal the wood.”
There was that magic “seal” word again. It triggered an idea. I bought some floor wax for a sealant and poured it over my sculptures. It worked perfectly, and because my ceramics teacher had impressed me with his teaching about Terra Sigillata, we all had wax protected sculptures before the semester ended.
Thank you for many lessons, Karl.