Amanda Redman on sat 5 jul 97
I am trying to pour some molds and after I pour them they always crack when I
try to take them out. How long should I wait to open them and could there be
something wrong with the slip that I am using. It is homemade slip. Made
from GoldArt highfire clay and water.
Also my clay is extremely wet. I am out of GoldArt and don't know how to dry
it out. Any suggestions?
Figred
John H. Rodgers on mon 7 jul 97
-- [ From: John H. Rodgers * EMC.Ver #2.5.02 ] --
I am not familiar with the GoldArt clay you have mentioned so my question
would be is it a dry clay body you have bought and mixed yourself.
Slip-casting clays are very specific in their make-up in that there are
chemicals called de-flocculants (sodium silicate, Darvan, etc) that you add
in specific ratios when mixing, to produce the desired viscosity without
having to increase the amount of water. To get this mix right on target you
must ( and I emphasize that) test the specific gravity of your mix, and the
viscosity. If it is not right on you can get all kinds of problems,
including sticking in the molds. All slipcast greenware will tear apart if
the molds are opened too soon. But if the clay is not "crisp" ie, specific
gravity and viscosity right on, no amount of waiting is going to help.
Westwood Ceramic Supply Co. (Bought out by Laguna Clay I think) put out a
neat little booklet called The Art of Slipmaking. The booklet describes the
whole process of making up slip. It should be a great help to you. It sure
helped me when I went through some trying slipcasting times. Another book
that deals with molds and has something to say about slipcasting is
"Moldmaking for Ceramics" by Donald E. Frith. This is the definitive book on
moldmaking and it can give you many insights into problems associated with
molds and their use. Laguna Clay, Axners and others carry it in their book
selections.
Just a few pointers about molds. Be sure they are cleaned thoroughly after
each use. I don't mean washing them , but be sure leftover clay is removed.I
prefer cleaning with fingers above all else. Rags, towels, etc., can damage
the mold. Allow the molds to dry thoroughly between each use. Depending on
the mold some can be poured twice in one day, but it is hard on the mold,
and increases wear. If you do this be absolutely sure you give it adequate
time to dry out before next use. Be sure the molds and the slip are at room
temperature before pouring. For my purposes 80F was always good. If the
slip is properly adjusted, and the molds have been cleaned and dried and
sticking is still a problem, other measures may be taken.
First, wash the inside of the mold with rubbing alcohol, using a china
bristle (boars hair) paint brush( disposable type) to scrub lightly with.
The alcohol will remove any oil present.
Second, using the china bristle brush, lightly wash with white vinegar.
This will kill or neutralize any soap scum or calcium stearate created in
the moldmaking process, both of which will cause sticking. Immediately flush
with fresh water and allow to dry thoroughly.
When you next use the mold, you may find that you must cast two or three
times, to condition the mold before getting a usable casting. If you don't
get a good piece the first try, cast again immediately. If the second is not
good, allow the mold to dry and try again. You should start getting good
pieces by the third try.
I know I havent't told you specifically how to solve your problem, but I
hope this helps.
John Rodgers - An Alaska Sourdough parboiled in the heat/humidity of the
Deep South
-------- REPLY, Original message follows --------
Date: Saturday, 05-Jul-97 09:23 AM
From: Amanda Redman \ America On-Line: (Figred)
To: CLAYART LIST \ Internet: (clayart@lsv.uky.edu)
Subject: troubles with clay and slip
----------------------------Original message----------------------------
I am trying to pour some molds and after I pour them they always crack when
I
try to take them out. How long should I wait to open them and could there
be
something wrong with the slip that I am using. It is homemade slip. Made
from GoldArt highfire clay and water.
Also my clay is extremely wet. I am out of GoldArt and don't know how to
dry
it out. Any suggestions?
Figred
-------- REPLY, End of original message --------
stevemills on tue 8 jul 97
Amanda,
If all you have in your "casting" slip is clay and water, that's where
your problem is, you need a defloculant in there and much less water in
consequence. I suggest you contact the clay maker and see if they have a
casting slip recipe for that clay.
Steve
In message , Amanda Redman writes
>----------------------------Original message----------------------------
>I am trying to pour some molds and after I pour them they always crack when I
>try to take them out. How long should I wait to open them and could there be
>something wrong with the slip that I am using. It is homemade slip. Made
>from GoldArt highfire clay and water.
>Also my clay is extremely wet. I am out of GoldArt and don't know how to dry
>it out. Any suggestions?
>Figred
>
--
Steve Mills
@Bath Potters Supplies
Dorset Close
Bath
BA2 3RF
UK
Tel:(44) (0)1225 337046
Fax:(44) (0)1225 462712
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