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Making bisque molds


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#1 clayshapes

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Posted 06 May 2013 - 05:31 PM

I make a lot of organic shaped bowls using acorn and other winter squashes as "molds" - that is, I form clay over the squashes to create the bowl shape I want.
It's not always easy to find just the right shaped squash at the market or farm stand (I get a lot of funny looks at the grocery store, as I examine the squashes very closely, sometimes for several minutes each!)..and when I find one I particularly like, I want to make as many bowls as possible from it. So I thought I'd try making bisque molds of some of them, to avoid losing the great shapes when the squashes eventually rot! (they last a surprisingly long time)
I've never made bisque molds before -- any advice about what kind of clay, and what cone to fire them to?
I've just made one today from the cone 6 porcelain clay I use -- (it's drying now) but I'm wondering if there is some reason to make these molds from low fire clay instead? I was planning on firing this cone 6 porcelain mold to cone 04, for the purpose of making the mold - but not sure that's right.
I have low fire cone 04 white clay, and I also have some very sturdy cone 6 black clay - which never warps..wondering if it might be a suitable choice for this task, aside from the issue of it possibly staining the white cone 6 porcelain I will be using to make the bowls, from these molds.

Thanks for any tips.
I've attached some pix of the what the bowls eventually look like when I'm finished embellishing them.

Attached File  lace bowls.jpg   315.86KB   124 downloads



#2 BeckyH

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Posted 06 May 2013 - 05:56 PM

You bowls are lovely. You can also make plaster molds, either as slump molds or as casting molds.

#3 clayshapes

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Posted 06 May 2013 - 06:09 PM

Thanks Becky -- I thought of making plaster molds...but frankly -- too much work!
I'd like to try bisque molds and see how it goes.

#4 bciskepottery

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Posted 06 May 2013 - 06:17 PM

Yes, plaster can be more work, but your bisque mold will shrink and then any bowls made from the bisque mold will shrink . . . so either you need an acorn squash 20% bigger than your intended bowl or you might want to rethink plaster.



#5 clayshapes

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Posted 06 May 2013 - 06:24 PM

Well, the mold is actually bigger than the squash - and size isn't that crucial. This isn't a museum quality project...I just want to try making a bisque mold.
Any tips?

#6 bciskepottery

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Posted 06 May 2013 - 06:36 PM

A few options. For a slump mold, you could press thick slabs over a squash (first treated with corn starch to help release the clay), let it stiffen up and then remove from the squash. Dry the mold slowly and bisque. From that mold, you could form slabs inside the mold to make bowls. For a hump mold, you would need to make the slump mold as above, but then use a fine clay to make an impression from the inside. You will need to be careful pressing in the slabs on the inside to prevent distorting the acorn shape. Another approach would be to sculpt an acorn squash in clay, let it stiffen up, then hollow it out. Again, slow dry and bisque.

#7 clayshapes

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Posted 06 May 2013 - 06:47 PM

Thanks - I've already made a hump mold from the squash, which is my preferred way of making these bowls. (I've made hundreds of them!) ...my real question is: what is the best kind of clay for a bisque mold -- low fire or cone 6. I am using cone 6 porcelain to make the mold at the moment, and plan to bisque it to 04. My original question was -- is it for some reason better to make the mold out of low fire earthenware?
I use the actual squashes as molds, so am quite familiar with the best way to use them as hump molds -- I don't use cornstarch - I cover them with very fine tshirt fabric to keep the clay from sticking to the squash. I've experimented with a lot of different methods and find this to be best for my process.
However, I thought the advantage of using a bisque mold was that the clay would come off cleanly - like a plaster mold. I guess I'll experiment with my first mold = after it's fired to see what works best.

Still looking for info about what kind of clay is best though -- and fired to what cone.
Thanks

#8 Min

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Posted 06 May 2013 - 07:44 PM

Hi, your pots are lovely! When I have made slump/hump molds with clay I used a fine grained clay and bisque fired it in the range of ^012 - ^010. You want it to be pourous to release fairly quickly. It helps to make them on the thick side so they are more pourous but not so thick that they crack while drying. If ^010 is too fragile for your form then try in the ^08 - ^06 range. Use your ^6 clay, not earthenware.

Min




Thanks - I've already made a hump mold from the squash, which is my preferred way of making these bowls. (I've made hundreds of them!) ...my real question is: what is the best kind of clay for a bisque mold -- low fire or cone 6. I am using cone 6 porcelain to make the mold at the moment, and plan to bisque it to 04. My original question was -- is it for some reason better to make the mold out of low fire earthenware?
I use the actual squashes as molds, so am quite familiar with the best way to use them as hump molds -- I don't use cornstarch - I cover them with very fine tshirt fabric to keep the clay from sticking to the squash. I've experimented with a lot of different methods and find this to be best for my process.
However, I thought the advantage of using a bisque mold was that the clay would come off cleanly - like a plaster mold. I guess I'll experiment with my first mold = after it's fired to see what works best.

Still looking for info about what kind of clay is best though -- and fired to what cone.
Thanks



#9 clayshapes

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Posted 06 May 2013 - 07:55 PM

Thanks Min -- I'll continue to use my cone 6 clay and fire a bit lower and see how that goes.

#10 Mark C.

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Posted 06 May 2013 - 08:37 PM

Bingo you said it (the clay would come off cleanly - like a plaster mold.)
Make it from plaster and that will work best.
Mark
Mark Cortright
www.liscomhillpottery.com

#11 SmartsyArtsy

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Posted 07 May 2013 - 03:07 AM

I don't have more to add re molds, but must comment on your bowls-- the are gorgeous. Very interesting and complex! And great use of color. Good luck with your molds.

#12 clayshapes

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Posted 07 May 2013 - 06:42 AM

Thanks Chris! Nice of you to say. I enjoy making them.




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