Jump to content
Sign in to follow this  
Textree

Choosing a clay for slip casting big pot

Recommended Posts

I've been kicking around of making a bonsai pot for a year or two. I don't have experience but Iv e done some research , found a place to fire it, and a ceramic store in town for supplies. 

I'm not skilled in pottery but I am pretty good with tools so I decided to make a wooden version the pot, use it to make a plaster of Paris mold, and pour a casting slip I prepare into it. 

My question is about the clay. The store has armadillo clays 2 or 3 are non absorbent enough at cone 6.  And a few other types and brands. They have a cone 5 porcelain (which can be fired to cone 6) , something called buffalo wallow with and without grog. They also have another brand of porcelain VPP (very plastic porcelain). If I am just turning them into slip is one harder to use than the other?  Is one more or less prone to breaking in the kiln? If it matters the pot would be big , like 18 inches by 15 by 5. The amount of plaster to make the mold, the amount of clay ill need, cost to fire the pot and glaze will make for an expensive project. Id prefer to pick something that I'm likely to succeed with. I like the idea of smooth vitreous porcelain but if its not going to work I can live with the other

Edited by Textree
Typo

Share this post


Link to post
Share on other sites

I decided to do this with half of the stoneware type , half of 213 porcelain and add some kyanite. I read up on mixing and deflocculating the slip. It seems like the difficulty is letting it dry very slowly. I have a garage freezer not in use so ill put it in there. 

Share this post


Link to post
Share on other sites

There are casting slips available that you can buy ready to go. I believe Standard Ceramic is one source. Otherwise, you can take any commercial body and make it into a casting slip. If you can't buy the clay body as a dry mix, then you'll need to buy moist clay and let it dry, then break it into the smallest pieces you can before making slip with it.

If you're just planning on making one or two of these pots, I would slab or coil build it. A mold that large is a big undertaking, and IMO not worth the effort for one or two pieces, especially if you're new to mold making and slip casting.

Here are my calculations for the volume of plaster you'll need, for #1 pottery plaster:

Your piece is 18x15x5 = 1350 cubic inches

If you make the mold 2 inches thick all around, that's 22x19x9 = 3762 cubic inches.

Subtract the volume of the piece from the volume of the outer mold, and that leaves 2412 cubic inches of plaster to make the mold.

You need 16.312 grams of plaster per cubic inch so 16.316 x 2412 = 39,354 grams of plaster. Divid that by 454 grams/lb and you get about 87 pounds of plaster. That's a big, heavy mold. You'll need almost 6 gallons of slip to fill it, so that's another 70 pounds or so added to the weight of the mold when it's full.

Share this post


Link to post
Share on other sites

That's almost exactly how I did that math so I'm on the right track. Thank you . like you, I figured out it will take 3 or 4 bags of plaster and was thinking 75 lbs of clay. The reason for the mold would just be I think I could make a nicer finished product that way. Whereas if I built it, it would look more amateurish . thanks again I might scale it down 

Share this post


Link to post
Share on other sites

18 × 15× 5 is large for a bonsai pot, they start to get hard to find at that size. But if porcelain / stoneware mix shrinks about 15 percent thats something like 15.3 × 12.75 × 4.25. I wouldn't want it much smaller than that

But youre right the more I think about it I wouldn't want more than 2 of the exact same thing .... The mold is somewhat wasteful 

Share this post


Link to post
Share on other sites

If you can make it work, it'll be an impressive pot that you could perhaps make more of and sell. I just think that if you're not an experienced mold maker and slip caster, a mold that big would be very difficult to make, and even more difficult to handle when it comes to the actual casting process. I would start with something much smaller, to familiarize yourself with the process, and see if going that big is something you're truly comfortable with. Things get complicated as they get big. I've made a number of molds, and taught my students how to do it, but I personally wouldn't feel comfortable tackling something that big without more experience.

Share this post


Link to post
Share on other sites

You will find a mold that big will have its own issues due to weight.Neil summed it up well in above post. Molds that large are problematic due to size and weight . Start small and work up. Same way with knowledge and experience.

One tip I can add if you do make a mold that big is sandwich it between to large round pieces of plywood that roll -that way you can drain it easier.

Share this post


Link to post
Share on other sites

I had roughly calculated the amounts of plaster and clay but not to see if it would be too heavy for me to tilt and drain, I was wondering how much I had to buy. It had not occurred to me that it will be awkward to tilt a 150lb  slab . Good point. I have some casters , a dolly , wheel barrow -  ill work out something 

The bags of plaster are 25lbs and about 25 bucks a piece at the depot. would I be better off with the stuff at the pottery store ? 

I guess I could make more pots once I get it to work. Or maybe ill gift the mold to someone at the pottery studio that can use it that way it isn't such a waste 

Share this post


Link to post
Share on other sites

A plug hole through the bottom of the mould, to coincide with one or more of the drainage holes of the final pot (bonsai pots do have drainage holes, don't they?) would mean you wouldn't have to lift the mould to pour the slip out. Just unstopper the hole(s), and the slip will drain out (assuming the mould sits over an appropriate void). Clean up the holes after setting, of course.

I've never tried it, but under these circumstances I would!

Also, I have used super fine architectural Plaster of Paris from builders' merchants to successfully make moulds, but I would hesitate to advise others to do the same simply in case it doesn't work for them. Potters' plaster is a particular formulation, or so people keep telling me, and I'm wrong - simply wrong - for using anything cheaper. (They never give me the money to buy the more expensive stuff, though.)

Share this post


Link to post
Share on other sites

That's a good idea about the drain. Ill think about that. And yes a bonsai pot must drain. I think the pottery stuff might even be less expensive than the sacks at the hardware store. Ill check. What I've read is that the mold picks up minute details from the positive so I guess I should caulk, sand, maybe paint or seal the entire wooden model ? 

Share this post


Link to post
Share on other sites

The mould will indeed pick up every tiny imperfection, and present it to the world forever more - although glazing will to some extent hide these. For this reason, I've always found that clay itself is the best material from which to fashion a model. But if you are more at home with wood, then yes, you need it to be beautifully smooth, and preferably sealed. It's only going to be in the mould for a comparatively short time, but you really don't want it absorbing water from the plaster, swelling, expanding fibres from the surface, etc. And you need to 'size' it so that it releases from the mould easily. @Chilly reproduced a chart somewhere on an older thread in this forum, explaining the best things to use as a release agent for any given model material. Do a search of older threads for that invaluable information.

One more thought - you'll need your mould to be totally dry before you start to cast with it. Don't underestimate the time it will take to dry out a mould of this size. In industry, they have dedicated mould drying rooms with a set of fans creating a draught across the room to facilitate drying - it still takes forever for large moulds.

Share this post


Link to post
Share on other sites

Plaster should be cheaper from the pottery supply place. Should be around $20 for 50 pounds.

Make sure your form is totally smooth and clean, and well sealed. Plaster will stick to wood pretty easily if there's any sort of grain texture. The more time you spend on the form, the less time you'll have to spend on cleaning up the clay piece. That's why for a one-off piece it might be faster and easier to hand build it rather than casting.

Share this post


Link to post
Share on other sites

Since you are making the original model out of wood, you might be better off using a hump mold and slabs vs. trying to master slip casting. I know you are not proficient in clay, but using a slab and hump mold is pretty easy even for beginners. There are plenty of youtube tutorials you can watch, but here is a quick read to get the idea. 
 

This way you are only paying for clay, glaze, and firing fees. You would probably need to use several large slabs since rolling a huge slab like that might be difficult. 

https://ceramicartsnetwork.org/daily/pottery-making-techniques/handbuilding-techniques/how-to-make-a-platter-using-a-slump-mold/

Share this post


Link to post
Share on other sites
8 hours ago, Textree said:

I had roughly calculated the amounts of plaster and clay but not to see if it would be too heavy for me to tilt and drain, I was wondering how much I had to buy. It had not occurred to me that it will be awkward to tilt a 150lb  slab . Good point. I have some casters , a dolly , wheel barrow -  ill work out something 

The bags of plaster are 25lbs and about 25 bucks a piece at the depot. would I be better off with the stuff at the pottery store ? 

I guess I could make more pots once I get it to work. Or maybe ill gift the mold to someone at the pottery studio that can use it that way it isn't such a waste 

Plaster sold at the Depot is not mold making plaster. The plaster you want is called #1 pottery plaster -is sold at ceramics suppliers and is in 50# bags not 25# bags.

You will find a huge difference in mold performance by using the wrong plaster.Use only new fresh plaster as it collects moisture quickly after its opened.

I now realize since all this is new to you you should really make some small molds and see whats involved as Neil suggested.

When it comes to say a 75# mols filled with say another 50#s of slip working something out at that point is called a disaster .

Make a small mold 1st learn to make slip and learn to slip cast all of these processes take time to learn. You will need slip tools as well to make your own slip or to adjust bought slip after its used.

Share this post


Link to post
Share on other sites

I didn't mean to suggest I'd wing it or cross that bridge when I get there. I will figure it out beforehand. Maybe I'm naive but to me this doesn't require skill so much as it does a plan. There's this movie I like with Anthony Hopkins and one of the Baldwin guys... They have to do something difficult and the Baldwin guy despairs . the difficult thing wasn't impossible , it had been done before.....Anthony hopkins says to him "what one man can do another can do!" 

Share this post


Link to post
Share on other sites

 

As for the tools for the slip - I have a heavy duty, 1/2 inch , corded drill and a whisk style mix attachment. I've used it to mix mortar and dry wall and, with a diamond hole saw attachment, it cuts granite so it should mix a clay slurry well enough.  

I'm going to pay the membership fee at the studio so I think they will let me fire whatever. I don't know if they let me use the pug mill for clay I carry in there (in case I wanted to hand build or use a slump mold of my mix) ill have to ask 

Edited by Textree
Duplicate text

Share this post


Link to post
Share on other sites

I guess I'm confused -is this a slip mold where you plan on pouring in liquid clay slip? Dump out when the wall thickness is right ? or a slump or hump mold where you lay a slab of clay into or over it?

Since you spoke about a drill mixer I thought this was a slip mold but now I'm not so sure?

Edited by Mark C.

Share this post


Link to post
Share on other sites

Slip mold - someone else suggested a slump mold or hand building. I planned on buying porcelain and stoneware moist , cutting it up and mixing it , letting it dry, and then remixing it with less water and darvan. 

Edited by Textree
Typo

Share this post


Link to post
Share on other sites
14 hours ago, Mark C. said:

Plaster sold at the Depot is not mold making plaster. The plaster you want is called #1 pottery plaster -is sold at ceramics suppliers and is in 50# bags not 25# bags.

You will find a huge difference in mold performance by using the wrong plaster.

As always, generalisations require caveats. The difference between No.1 Pottery Plaster and the architectural quality Plaster of Paris available in builders' merchants for mouldings, cornices and general ornamental plasterwork is not that great - the main difference is in dry strength, where No.1 Pottery Plaster is perhaps 15% - 20% stronger - and even that assumes that the plasters are mixed properly, which of course at a craft level most people don't really do.

I have a number of moulds, both slip- and press-, made from architectural grade Plaster of Paris which have lasted many years, and are still in great condition. In my part of the world it's a much cheaper option, and the difference is slight enough that it is, as they say, a no-brainer.

Share this post


Link to post
Share on other sites

Hi Textree

I would definitely make a first try with a hump mould.  You need to make a "master" anyway, before you can make a slip mould, so you might as well.

Clay will release from untreated wood but I usually line with thin plastic as it makes it easier.  Place your hump mould on a block so the rim is not touching the table, then you can get to the rim of your pot and cut it off level.

I have made many large bonsai pots, I've used all methods except throwing.  Slip-casting, slump, hump, coiled free-form, coiled inside a biscuit tin, slab-sided, bricked.  Unless I was going into production I would not be wanting to make a slip-casting mould.  Too much time and effort, too heavy when empty, far too heavy when full.

The cast-iron Owl below is 12 inches tall, and the mould is too heavy for me to lift when full.  I can just about slide it around on the table, when ready to tip.  If you do go for a mould, a plug for emptying is a must.

large.IMG_2079.JPG.f3a89c8169aa644b8992a

Edited by Chilly

Share this post


Link to post
Share on other sites

Maybe ill try that chilly . thank you. Even if its 150 lbs full I'm pretty sure I could tilt it and drain it. But just in case,  I have a small 4 wheel dolly. Its flat. Similar to the picture below. I can fasten the block to it and the wheels should make it a little easier to tip. 

One reason the slip casting thing appealed to me is that it requires no attaching pieces together. I've read that those are the places that fail in drying and firing , the ones you glued together with slip which may not dry at the same speed. 

If the biggest problem with the slip mold is that its big and heavy then I'm cool with that . I'm a big dude. I was more worried about it being a waste if I didn't want several nearly identical pots. Its not cost effective but that's ok. Early on in my research I figured out saving money making your own bonsai pot is like trying to save money growing your own tomatoes . 

Sooner or later ill try some other techniques. I initially wanted to try the wheel but people here and elsewhere told me that is a lot harder than it looks. 

Thanks again everyone for the advice and information . 

H-3318.jpg

Share this post


Link to post
Share on other sites
On 2/20/2018 at 4:01 PM, Textree said:

18 × 15× 5 is large for a bonsai pot, they start to get hard to find at that size. But if porcelain / stoneware mix shrinks about 15 percent thats something like 15.3 × 12.75 × 4.25. I wouldn't want it much smaller than that

Is this going to be a rectangular bonsai pot? If so it wouldn't be difficult to make from slabs. I've made a few really big planters this way. Thick slabs, like 3/4" thick made from a super coarse clay, let them stiffen up  then miter the joins and slip / score. No molds or forms or special equipment needed.

Share this post


Link to post
Share on other sites

What kind of clay Min? I'm worried about "super coarse".  A nice bonsai pot should be vitreous or at least really close.  Its wet and exposed to freezing and thawing and triple digit heat for months ...and porous pots will absorb water and break in time.  truthfully for me , in texas, this is a vain consideration because it just doesn't get cold enough for that to be a real issue. I have cheap terra cotta pots that have lasted a decade. But if I'm going to the trouble to do it im going to make it nice and the best way. Stoneware and or porcelain. And ill write my name on the bottom . and my grandkids will marvel at my ingenuity and craftsmanship. ;)

Share this post


Link to post
Share on other sites

 I really doubt the clay I used would be available to you, F78G from Plainsman Clay in Alberta.  Haven't used any clays from Armadillo and don't know if they are an option for you but their Buffalo Wallow looks like it could be tight enough with a posted porosity of 1.73  There should be someone on the forums who knows about their clays and/or other ones in your area. I would look for low porosity figures, some sand or grog for strength, the colour of the fired clay. I would just buy a bag to start with and run your own porosity and absorption tests on it before making the pot. Compress the clay slabs with a rib to push any grog or sand into the clay. If the unglazed clay is too rough you can sieve some slip made from the same clay and right after rolling and compressing the slabs brush a couple coats of slip on the slab. 

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
Sign in to follow this  

×

Important Information

By using this site, you agree to our Terms of Use.