Making casting slip from reclaim clay

[JJ Bedore]

I have made casting slip from reclaim clay. A couple of questions:  the texture of my casting slip is heavy whipped cream. The specific gravity is 168. I'm not sure how to get the viscosity to "cream state" so I can pour it while increasing the specific gravity to between 170-180. Do I need a deflocculent (I have Darvan)? Or does reclaim clay have enough Darvan already in it that I shouldn't add more?  Do I even need to measure Specific Gravity with reclaim clay?  I used a little Darvan on a test batch (not my whole bucket) and poured it into a cup mold but it took hours to dry and was very thin. I'm missing something here.  Thank you for any help you can provide. 

Edited March 22, 2023 by JJ Bedore

[Min]

Hi and welcome to the forum!

Just to confirm,  the casting slip you made is from reclaimed pugged clay? If so then yes you will need to add a deflocculent such as Darvan. To increase the specific gravity you will need to evaporate some of the water off the slip. First deflocculate a test cup amount then let it evaporate to get the specific gravity.

Be aware that not all pugged clays are suitable for turning into casting slip due to the salts and particle sizes of some clay bodies.  Very little Darvan is necessary, just add a drop at a time to a small test amount until the slip thins out but don't overdue it. Flow of casting slip is also really important, when you put your hand into a bucket of properly deflocculated slip it should web between your fingers when you pull your hand out. I have simplified the process here, specific gravity, deflocculation, viscosity and mixing time are all important. There is a good article explaining this in much more depth here if you need it.

[Mark C.]

Sounds like you are reclaiming casting slip clay???  If so its got darvan in it already

You should do a viscosity test  (ford cup)on your store bought slip and then on your reclaim slip and make sure they are about the same

[Jeff Longtin]

Hello JJ,

In my experience reclaimed casting slip is not like new casting slip. Castings come out thicker and they take more time to set up. (The clay also tends to stay soft much longer.) As a result I try to mix my reclaim with new whenever possible. (50-50 isn't bad but 25% reclaim and 75% new is much better.)

Trying to cast pots with reclaim slip and expecting it, to be like new slip, only leads to frustration. (Been there done that.)

Knowing the kind of clay you have would help greatly.  All three clays, porcelain, stoneware, and earthenware, need to be deflocculated, but they can be deflocculated with different substances.  Darvan 7 is common for porcelain, but sometimes  sodium silicate is still used when manufacturers want to save money.  Soda ash can also be used.

Generally speaking I don't add deflocculant to reclaim. (As its already in there.) But different clays respond differently.

Your specific gravity at 1.68 is good. It indicates you should have enough water in your slip.  However the gel state could indicate a need for more deflocculat but before I suggest that more info would help.