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Dick White

Terra Sig question

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A technical question for the terra sig cognoscenti here.

Generalized background (nothing new here, just setting up the question to follow): T.S. is made by mixing some clay with an amount of water and adding some deflocculant to cause the clay particles to begin settling out. The larger particles will settle first and the finer particles will remain suspended longer. After a time, the upper part of the suspension with the finer particles is decanted as the T.S., and the sludge at the bottom consisting of the larger, settled particles is tossed out.

Issue: As the decanted T.S. rests in its storage container awaiting its next use, it continues to settle out with a much thinner layer of sludgey clay on the bottom of the jar.

Question: Would it be appropriate to reflocculate the finished T.S. to retard the further settling of the clay during storage? Would this change the working properties/procedures in any material way?

Thanks for all your wisdom and insight?

dw

 

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Dick,
Once you have separated the TS from the large particles there is no need to worry about flocculation/deflocculation of the material.  Just adjust the concentration of particles to the value (usually measured by specific gravity) that you like.  Vince Pitelka likes 1.5 specific gravity. 
 
Direct answer to your two questions:  1.  No.  In still water, the visible particles will settle over time, regardless of the flocculation/deflocculation.   2.  This depends on what additive is used to adjust the flocculation/deflocculation condition.  Since the most common reagent is sodium silicate, or soda ash, you will be adding flux to the clay particles and will lower the temperature at which the particles become a glaze. 
 
I often dry the TS and then crush it to powder and store the powder in a candy tin until I need to make a TS.  Then just let leftover dry and put back into the tin  until next needed.
See Vince's TS web page: https://sites.tntech.edu/wpitelka/terra-sigillata/       A very thorough document.
LT
 
Edited by Magnolia Mud Research

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Thanks Magnolia, I am familiar with Vince Pitelka's T.S. page, and have been using that recipe for a number of years now. Here is the practical basis for my questions. My community studio will be having a naked raku workshop/firing in about 2 months, led by the inestimable Ray Bogle. In preparation for that day of fun and games, I made a large batch of OM4 T.S. for the general use of the studio members, resulting in about a gallon of finished sig. I've divided it into several jars, some will be colored with various stains or oxides, others will be left plain white. All are continuing to settle, with the thin layer of sludge sometimes being difficult to stir back up into the T.S. without remaining lumpy. And in the stained ones, the stain drops out first (as expected) but then becomes trapped by the overlying layer of new sludge. Also, if the specific gravity was correct (and yes, I too like 1.15 for this OM4 sig) and some portion of the solids settles out and is not thoroughly scraped up by a less-fastidious student, then the S.G. of the remaining sig will be lower, possibly too watery. Because this will be an intermittent and on-going activity for a few more weeks, I can't do the dry storage technique, so I am wondering if refloccing it with a dash of epsom or vinegar as we do with glazes will keep things floating longer without simultaneously causing the sig itself to become unworkable?

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I don't use TS with stains, so have no direct experience.  However based on simple fluid mechanics of particles settling in water, the general situation is as follows:
The density of stains and oxides added to the TS are significantly more dense than the particles of the clay and will sink much faster than the clay particles unless they (the heavy particles) are significantly smaller than the clay particles -- which in my experience is unlikely. 
Conclusion: The stains and oxides will always be hard to keep in suspension.
 
I am inclined to use a high speed blender to re-suspend the materials just before using adding the "extras" to the slurry. 
 
The OM-4 TS we use at school works fine just shaking the gallon jug just before using the TS on naked Raku pieces or foot rings of bowls and cups. 
 
Given your observations, I would also try on a small sample adding vinegar, etc. to see what happens.  It the treatment  helps, go for it. 
I'd also make a backup source of pure TS for backup. 
LT

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Dick, I usually just stir the TS after it has been sitting in between uses. Adding Mason stains is a different story. They tend to settle quickly from my experience. If you can keep the stains in solution while using, no problem.  I like Darvon 811 as a deflocculent.

Marcia

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