Thickness Of Vases From Slip Molds?

[missholly]

i poured my first few slip molds last night.

two vases, they came out beautifully.

 

i cant tell if the walls are too thick to fire.

i left the slip in for a few hours, poured it out and let them sit over night.

 

what are the guidelines? and is there a way i can test it? (stick a pin tool in it?)

[MMB]

Well theyre molds so you can always redo them. Slice one in half and take a look at the thickness. Im not a master at molding but Ive never left the slip in there for more than 30-45mins max ( also depending on the mold). Just enough to get a decent thickness. If Im stressing the mold by doing multiples, which I cut it off at 3 and let the dry out, I tend to add an extra 15 mins after each since the mold can only absorb so much water. Doing that though really cuts the lifespan of the mold down a lot. I cant imagine how thick one could get after a few hours, but if youre going to go thick the only thing I would make sure of is that you give it a lot of time to dry and do a long soak before turning up the heat in the kiln.

 

I was reading about a lady that sculpts and she fires wet and relatively thick but soaks it at 180 for 4 hours.

[missholly]

wow. thanks for the advice.

i didnt even think about letting the mold rest between pours.

 

now im not sure if i should ditch these and start over, or go ahead and decorate and fire.

 

do you recondition slip just as you would regular clay? as in just adding water?

[MMB]

Yeah, the plaster just get more wet after each cast and after any use the plaster mold will eventually start to loose its "sharpness." Detail and fine edges will fade or become more round.

 

I have reconditioned mine. Some advise against because you can throw off your proportions of water/clay/deflocculant. My slip is usually a little imperfect whether it be a little thick or just right. I would advise if you do recondition cast items make sure they have time to disperse and always blend the crap out of the bucket to insure you dont get clumps. I usually make sure its bone dry and Ill crush it as much as possible before tossing it back in. Large pieces seem to never want to fall apart in the bucket. I wouldnt add any extra deflocculant being that you only need a tiny bit to begin with which should still reside in the clay pieces.

 

Like I said Im not crazy experienced but havent had too much trouble myself so far with what Ive dabbled with.

[missholly]

how thick should the walls be? im thinking these walls might be 1/2 inch?

[MMB]

I usually cast mine around 1/4 inch but it does fluctuate sometimes. Really though its all in what youre going for. If it is going to be larger of course you would want it a little stronger for the strength. If its smaller than you can thin it out a bit. Also to think of weight. Then again the thickness and weight really depend on the person whos making it. Everyone has their own little comfort zone. Personally I like mine thicker so a lot of the times the fluctuation I mentioned is more than 1/4 inch. I do this because Im experimenting with pitfiring and its a rough process on the pots.

[missholly]

im just afraid theyre going to explode in the kiln. should i maybe just fire slower?

[bciskepottery]

For thick pieces, like scuptures, our studio makes sure the pieces are dry and then does a 6 hour pre-fire of the items at 200 degrees to give enough time for moisture to be removed. The key is to let thicker pieces dry -- don't rush the piece to firing too soon. We also follow the slow bisque program on the kilns.