Wet Grinding Bisqueware?

[cemoore]

I am new to slip casting but not to mold making and other mediums. In another medium (exposed aggregate concrete), I've made custom wholesale products that required me to make a larger-than-spec round item from a mold, then grind it down to an exacting diameter (within +/-.02") standard. With that medium, grinding of 1/32 to 1/24" was required anyway to expose the decorative stones in the  concrete body. So, I'm thinking as bisqueware (in my case stoneware) can theoretically be sanded, are there opportunities to make a thicker walled item and grind it down after bisque firing to the required diameter with wet diamond pads? If so, how thick is too thick? Are solid castings an option? I'd also have to take into account any thickness added by glazes when grinding. Just thinking out loud, would love to hear from slip casting ceramicists on this idea. 

PS. I think the answer on the keeping the final item within spec is that the amount of additional shrinkage during glazing might be too difficult to pinpoint or distort the shape?

 

Edited July 2, 2023 by cemoore

[Hulk]

Interesting questions!

To try for +/- twenty thousandths (a fiftieth of an inch), perhaps taking off the last little bit after the final fire - for unglazed ware - would be an option?
Then the additional shrinkage wouldn't be a factor.

I'm curious what you're making.

Where two (or more) parts have to fit together, one can get fairly close by measuring the pieces when they are in the same state, where it helps that the pieces are about the same thickness, have similar curves, were worked similarly, there's more than one each to pick from, and there's experience, specific trials, and notes to draw upon.
Fitting together may be a simpler problem that meeting your dimension spec.

As for thickness, too thick can happen quickly. Getting the ware to dry well - evenly - without cracking, distortion, and then firing so the thick bits are bisqued all the way through, again, without defect, may be challenging. I'd suggest adding only the amount necessary to allow for grinding to spec.

Any road, given everything is repeatable, you might be able to get close.

[cemoore]

Thanks Tom/(Hulk)! I like your idea of unglazed and hollow but with enough extra thickness to account for removal. It seems there would be so many variations, cutting it down seems the most predictable and practical. Without a glaze, the body would have to be semi-vitreous or vitreous. As to what I make from concrete to the narrow specs - they are OEM parts that go in a bathroom/damp setting, that are attached to cast metal parts. The part shape looks about like an ice hockey puck - nothing truly fancy in shape.  At the end of the day, while my 3d printed models looked round to my eyes, they were slightly off using caliper measures (eg, not perfectly round). A partial solution to that might be spending thousands on machined models, but that's not a financial option. As well, the casting process is always introducing some variations. Milling them after casting made them beautifully round and within spec. As they were small I ended up using a drill press and attaching into a hole on their underside a blank drill bit with a flexible plastic gasket to get tight on the bit. I used a velcro-backed flexible diamond hand pad affixed to a 3d printed arc'd pad holder to get the curved sidewall surface to 90 degrees. A sort of poor-man's milling. Just thinking about that project in ceramic but also future projects where I might need to meet these kind of specs and ways I might be able to machine/grind my own designs.  

Your input is greatly appreciated! Always learning.

 

[Hulk]

Interesting, thanks for the detail.

Check back, you may get more input!

[PeterH]

Casting bodies don't need to be very plastic, and can have comparatively low shrinkage.

https://digitalfire.com/glossary/slip+casting
Slip cast ware can dry-shrink as little as 1.5% (compared to 6%-8% for plastic stoneware bodies).

https://www.potterycrafts.co.uk/Products/pottery-powdered-clays/P3101
Shrinkage (w-d) 2 - 4% & Shrinkage (d-f) 6 - 8%

So maybe 4% for bisque to fully fired?

However you could high-fire  to maturity, grind, and then either high or low-fire to glaze -- so zero shrinkage. Bone-china is traditionally fired high-bisque low-glost (it needs to be supported when fired to maturity).

Glazing a mature body presents problems, as it isn't porous, but is probably doable (AFAIK industrially various additives are used).

PS Solid casting is possible, unless the gradual build-up of clay obstructs the inflow of fresh slip. A hockey puck seems a pretty undemanding shape.

Edited July 3, 2023 by PeterH

[cemoore]

Thanks Peter, I read the links and will check out bone china. Sounds like a material that's outside my current skill level (everything is at the moment with slip casting), but maybe aspirational. I like the idea of being able to glaze after grinding and knowing there won't be discernible shrinkage. 

[PeterH]

21 minutes ago, cemoore said:

Thanks Peter, I read the links and will check out bone china. Sounds like a material that's outside my current skill level (everything is at the moment with slip casting), but maybe aspirational. I like the idea of being able to glaze after grinding and knowing there won't be discernible shrinkage. 

I was definitely NOT suggesting that you used bone china, it warps/slumps too much. Just using it as the classical example of high-bisque low-glost approach. If you do read up on it I would concentrate on how they get the glaze to stick to a non-porous body: pre-heating, additives, ...? 

Will you need the puck to be glazed on all surfaces? If so using stilts is one option, another might be to  glaze the "bottom" in an additional lower-fired glazing.

[PeterH]

Some ideas on glazing non-porous bodies in this thread.

[Orbit]

I high bisque, “low” glaze some of my very thin and fragile porcelain paperclay sculptures that need props to get to vitrification without slumping or warping, so I bisque to cone 8 and then glaze at cone 6, you only need to glaze at least two cones lower than you vitrify your clay at. In order to get the glaze to stick, it is actually pretty easy. Warm up the ware in your kiln or oven or with a torch (being careful with the torch not to stay in any one area too long or you could crack it) and I add CMC and sometimes gum arabic to my glazes. If I am absolutely having a hard time getting the first coat of glaze to stick on I will spray hairspray on the ware or add a little Karo syrup (anything that is sticky and will burn out early) into the glaze. Once you’ve got one coat on, the rest of the coats will stick to the first coat and you will have no problems. I personally spray, but this works with dipping or especially brushing as well. I also use bone china, but like the others wouldn’t recommend it to start with.