Post #378070 by Babalu on Sun, May 4, 2008 8:38 PM

Hey Grog,

Love the humidity question...I can tell you have done this before :) It takes longer for clay to dry in humid conditions verses dry conditions. Those of you who live in places like Florida, where you can cut the water in the air with a knife, are going to find that you have a harder time drying work than those of you living in say...Denver, where the air is very dry. I grew up learning about clay in Colorado...Things dry super, super fast there. Not sure about the temperature part of your question Grog, but I can imagine that you would not want to try doing a casting where it might be too cold...no freezing temperatures.

Time it takes for the slip to harden in mold:

• I've talked to a few people how have different times for this, but I kind of like my mugs to be just a little thicker than the norm...they just feel juicer to me. I let the slip sit in the molds for about an hour before I pour it out.

Time to let the slip cast dry in the mold:

• Generally, when you see the clay start to pull away from the plaster a little, it's time to open the mold. I have kind of fallen into a routine with my mugs. I fill and pour them out right before I go to bed at night...I let them sit inverted all night...In the morning I open the mold and remove the slip cast.

When is it time to dry molds:

• I usually sit them out in the sun to dry for a day after about 4 or 5 pours. I have heard that some folks like to dry their molds out in an oven at about 200 degrees f. for an hour or so too. I have also heard of folks popping their wet molds on top of a hot kiln to dry them out

Richard Burket is head of the Ceramics Department at SDSU...cool dude. Took one class from him when I first moved to So Cal...here's what he has to say about the topic...

Richard Burkett - SDSU 1993

1. Make sure that new plaster molds have dried sufficiently. It
   will take 3-4 days at the least for them to be dry enough to use,
   unless you leave them in the sun all day, or dry the molds in a
   warm place. Be careful with forced drying, as plaster will start
   to deteriorate above 130° F or so. Make sure the mold is clean and
   free of all oil, mold release, or grease.

2. If the mold is completely dry (no dampness whatsoever) it
   should be slightly dampened before slip casting. Do this by
   running it quickly under water (or dip it in a bucket of clean
   water) about 5-10 minutes before you want to pour the mold. If the
   plaster is not slightly damp, the slip will cast with varying
   density (harder on the outside where the dry plaster quickly draws
   the water out/softer toward the inside) and cracking will be more
   likely. Air bubbles and porous casts are another possibility when
   casting in a dry mold. A damp mold will also make dusting more
   effective (see step 4).

3. Dust the mold if necessary. Talc is fine for lowfire casting
   slip. Silica is better for porcelain casting slip, as the talc
   will melt on the surface during firing. Use a pounce made from
   thin cloth to lightly dust the interior of the mold. Too much dust
   and the fine details in the mold will be lost, and the casting may
   come loose prematurely and collapse. This dusting allows the
   casting to release from the mold earlier and more easily than it
   would otherwise and is not always necessary with simple molds.
   NOTE: Wear a dust mask and use the glaze spray booth for proper
   ventilation when using Talc or Silica dust to avoid dust
   inhalation.

4. Fasten the mold together tightly with large rubber bands,
   pieces of rubber innertube, or heavy string tied, twisted, and
   wedged snugly about the mold. Be sure the mold is quite securely
   fastened, as the slip pressure inside even moderately-sized molds
   is surprisingly great. With very large molds, be sure you will be
   able to drain them when filled and heavy with slip.

5. Make sure that the casting slip is well blunged (mixed),
   free of lumps, and of the proper consistency.

6. Prop the mold upright with a little clay if necessary, and
   have a bit of clay ready to patch leaks. Pour the casting slip
   into the mold quickly, filling the mold to the very top. As the
   slip level drops in the mold, keep refilling the mold with casting
   slip. If the mold has a short sprue, fixing a large funnel into
   the sprue with a little clay may cut down on number of times you
   have to refill the sprue with slip. Do this for 5-15 minutes. The
   actual time will depend on the mold, slip thickness, and how thick
   of a casting is desired. Check the thickness by pouring a small
   amount of slip out of the mold and cutting through the casting
   with a needle tool.

7. Drain the excess casting slip from the mold. You may have to
   use a knife to open the sprue if it has cast shut. Do not to cut
   into the plaster! After most of the slip has drained out of the
   mold, prop the mold so that the sprue is down, and all remaining
   liquid casting slip continues to drip out. A good method is to
   place two sticks across the top of a bucket and prop the mold on
   the sticks to that it drains into the bucket. Allow the mold to
   drain upside down like this for at least 15-20 minutes. Avoid
   shaking the mold! This may loosen the casting from the plaster
   and cause it to collapse.

8. Allow the mold to dry until it can be disassembled without
   tearing or collapsing the casting. This drying time will depend on
   the wetness of the mold and the thickness of the casting. Typical
   time from pouring to removal from mold is 1-4 hours, occasionally
   it takes overnight.

9. Clean up!

Richard's web Site - http://www.richardburkett.com/