Published May 29, 2023
My first exposure to the word siphon was in some old movie in which the outlaw characters were siphoning gas so they could continue their getaway. Little did I know that some day, I would be using that process in a more noble way—in ceramics! Siphoning is a skill that potters who are interested in using terra sigillata need to learn.
Learning how to siphon allows potters to extract the "good stuff"—the terra sig—out of a settled mixture of clay, water, and a deflocculant. In today's post, an excerpt from Terra Sigillata: Contemporary Techniques, Rhonda Willers explains how to siphon terra sigillata. –Jennifer Poellot Harnetty, editor
Siphon Definition: A tube used to convey liquid upwards from a reservoir and then down to a lower level of its own accord (without a pump). Once the liquid has been forced into the tube, typically by suction or immersion, flow continues unaided.
How to Siphon
To prepare for siphoning the terra sigillata, have a 5-gallon bucket positioned below the original bucket containing the settled and undisturbed mixture. Have a second smaller container, such as a 2-liter Mix ‘n Measure, to the side of the empty 5-gallon bucket, keeping it lower than the settling container. This second container will catch the initial wastewater from the siphoning process (see 3).
Fill the clear plastic tubing with water from the sink. As the tubing is filled with water, maintain a position with the tubing that allows it to be as full as possible. Ideally, there is only ½–1 inch of empty space at either end of the tubing with the remaining space filled with water. In a larger sink, hold the tubing in a U-shape to achieve this (1). Or, in a smaller sink, wrap the tubing into a circle a couple of times to keep the levels correct to fill the tubing. Cap both ends of the tubing with your thumbs (2).
Before placing one hand with the tubing in the settled mixture, observe where the top of the sludge layer appears. During the siphoning process, hover above this layer. Bring one hand, with your thumb still blocking the opening of the tube, slowly into the settled mixture. Move this hand down into the settling container until it is hovering above the sludge layer. The other hand should continue to block the opposite end of the tube with your thumb. Move this hand so the opening of the tubing is over the second smaller container (3). In the siphoning process, the wastewater will come first. Be prepared for a quick gush of water that quickly transitions into terra sigillata.
Once the hands are in position, first remove your thumb from the bottom of the tubing, which is above the smaller container (4), very quickly (a matter of seconds) followed by removing your thumb from the top of the tubing, which is inside the terra sigillata layer. Keep the tubing over the smaller container until all the wastewater is out of the tubing and the terra sigillata from the middle layer fills the tubing.
How to siphon using waste water
When the wastewater is out of the tubing and the terra sigillata starts to come, do one of the following:
Option 1: Cap the bottom of the tubing with your thumb again and quickly move the tubing to the empty 5-gallon bucket then release your thumb to allow the terra sigillata to flow again (5). Note: This method leads to the least amount of mess in the studio, but sometimes the siphon flow stops and it is necessary to start the siphoning action again.
Option 2: Quickly move the tubing, with the terra sigillata still flowing, to the 5-gallon bucket.
Continue to allow the terra sigillata to flow into the 5-gallon bucket until all of the terra sigillata has been removed from the settling container. Throughout the process, be mindful of the sludge layer and remain slightly above it. Avoid transferring too much of this heavy material, which will settle out from the terra sigillata later and cause potential application issues.
After the terra sigillata is removed, the sludge at the bottom of the bucket can be discarded, as it does not offer a significant use beyond this process (6). Note: There is a potential use for the sludge to create thick slips, but experimentation and testing would be needed.
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