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dom92

Can't get cup out of slip mold after changing clay type

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Hi All - New to the forum but not new to ceramics.  I've got a strange problem I'm hoping somebody here can help with.  For several years now I've been using a locally purchased Porcelain ^6 slip to make coffee cups and mugs in two part slip molds.  My normal workflow was to pour the slip, wait about 15 minutes, then pour off the excess.  After about 2 hours or so I'd open up the mold and pop out the object to let it air dry.  These steps have worked well and I've never had any problems to speak of. 

Yesterday I went to the shop to get some more slip and they advised they had discontinued my favorite product.  They recommended what they said was an almost identical Porcelain ^6 slip, except this stuff was "translucent."  I thought I'd give it a try.  The problem is there is clearly something very different with this slip that has thus far made it impossible for me to get objects out of the two part mold.  I tried my normal workflow and everything went fine until I tried to open the mold after 2 hours.  The cup split in half down the mold seam when I opened it.  Minor glitch, I thought.  I tried another one but then let it sit for 4 hours before trying to open the mold.  This one split down the mold seam as well.  So right before bed last night I tried one last time and decided to let the mold sit overnight before opening.  When I awoke this morning I'd found the clay had dried enough it was starting to pull away from the sides of the mold but now there was a 2" crack extending down from the lip of the cup.  Clearly this new clay doesn't like me.  :)

I must be missing something here.  I've never used a "translucent" slip before, so maybe I'm making a major mistake in my workflow I'm not seeing.  I'd appreciate any advice others may have. 

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Sounds like its the new slip. I never have heard of translucent slip. Usually when you have issues with molds its because they are not dry enough.-Sounds like you know this process.

When they tear at the seam the clay is still to wet-when they dry and are loose in mold it means it was left to long in mold.

If I where you I would track down your favorite slip product and buy that.I do not know where that is -you can buy slip in dry form or in 5 gallons or 1 gallons in wet form.

Or you could make your own slip and this will after you dial that process in will never happen to you again.It will be cheaper as well.I do not have a cone 6 slip recipe 

Edited by Mark C.

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To obtain translucency; two primary changes in clay chemistry has to occur. The first being high purity grades of kaolin are required.The second, at cone six flux levels have to increase by 15-20%. Giving the conditions you are describing; I will go with 15-20% increase of nepheline syenite. Why? Because sodium in an aqueous solution will cause a hydrophobic reaction: meaning it has the tendency to repel water. In your case: rapid drying = cracking. Pending the sodium level, there may be a slight exothermic reaction as well. Anyone who has poured a plaster mold has felt this exothermic reaction when as it sets up: the mold gets warm. 

Wait 30-60 minutes after you pour: is the mold slightly warmer? If so, simple proof of high sodium content. If this holds to be true: then you need to pull the form much earlier. Only experimenting with the time will tell you when. High levels of sodium also means higher PH levels. The higher the PH, the faster it will dry. Higher PH will also effect plasticity, which will also cause cracking. You can try this simple (possible) fix: add 3 drops of vinegar to one cup of slip: stir then pour. Check the form for improvements. If cracks are less, but still present: add 5 drops of vinegar.in essence, you are Titrating the PH level back into the 8.35 range; where porcelains should be.

Lastly, if Nep Sy is the flux of choice (usually is because it is the cheapest flux available) then you will have other issues arise in the near future. Nep Sy has 14-20% soluble salts that will build up on your molds in a short time. This build up will degrade your mold much faster than the norm.

if you have other suppliers available to you, mark's advice of finding a new slip is a good idea. All depends on how much time you want to spend dialing this new slip in.

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I appreciate the advice, everyone.  It's definitely something about the chemistry of the new slip.  I scrounged up enough of the old slip from some used barrels to make another pour.  It turned out perfectly, just like it always has.  I may give the vinegar fix a try and see what happens. 

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They've stopped making it in any quantities that are useful to me.  I only buy my slip in 1 gallon jugs because it'll take me 6 months to a year use it.  I believe they said I could still get it in 5 buckets, but I thought that was simply too much to deal with.  Not only in having a single batch of slip sitting around for years but also just in having to heft that thing around.  Given the problems I'm having with this new slip, however, I may need to reconsider things.  My local ceramic supply store is pretty big and provides the materials to most of the local craft shops and studios in our metro area.  All the clay materials they sell they make themselves.  

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