Slip cast porcelain vs. stoneware cone 6

[Rareearth]

I am just starting into the world of slip casting after throwing for a few years. I have routinely used b-mix stoneware that I fire to cone 5/6. I am trying to figure out if a stoneware slip or a porcelain slip will be the most durable. I’m still firing to cone6. I am making many many plates for a high-end caterer, so I just need them to be strong, but I figure slip cast will help with uniformity over throwing. Suggestions?

[Bill Kielb]

This is a tough question

Strength in ceramics is interesting in that porcelain could be considered harder / more dense and therefore stronger but things that are harder tend to be more brittle as well. Ceramics tend to be strong in compression and weak in tension. Steel is about equally strong in both so it makes for a very durable predictable design material. 
My thought without trying to determine the ultimate strength properties of the two materials it’s probably more practical to respond to the stresses placed on the ware and how to best respond to them. So torsion, means thicker  sections and closed sections. Axial again generally means thicker, impact ........... well ...............

Anyway - use sturdy looking shapes that are appropriate thickness for their use (not too thin). Minimize imperfections which casting will help but if you create decorative voids then round chamfered rather than square and sharp edged, Speaking of imperfections, stoneware can have larger particles than porcelain which can be considered possible imperfections .........  Geez! 

Double anyways - try and use a durable glaze with similar fired COE as that of the claybody  as a claybody slightly in compression is significantly stronger than one that is not.

Triple anyways - a durable glaze goes a long way as wear and tear on it will take a toll, especially dishwasher, utensils, heck just stacking will wear the glaze.

How will you know this? Experience and time will reveal less durable products.

General thought, fire to fully vitrify and for low absorption as well. If it’s a cone 6 clay fire to cone six. Firing a cone six body to cone five almost always produces a less mature product.

[Callie Beller Diesel]

I think a lot of restaurant ware is made by jigger/jolly systems, which involve soft clay being pushed over or into plaster molds. It lets you use a sturdier clay formulation, one you’re familiar with, while still allowing you to make identical pieces rapidly and in quantity. You can make a simple enough setup in a studio pottery with a wheel and some bats that are keyed to the wheel head and a simple arm that drops the template down. The larger industrial machines are a bit fancier, but operate on the same principal and go faster. 
https://pureandsimplepottery.com/pages/jiggering
 The one example cited on this page of Bob and Connie Pike’s work: I had the good fortune to be mentored at one point by this couple. I even got to run their jigger for an afternoon when they were still making some pieces with this method. (It’s pretty fun!) The goblet and mug in that image are probably thrown, but the bowls and plates are jiggered. They’ll have been made of the same clay, which was either one of the Plainsman porcelains (P600?) or Laguna B-mix. They’ve used both, but my memory is hazy on when they switched things up. 

[Chilly]

Slip-casting plates will be incredibly slow.  As @Callie Beller Diesel says, commercially they are made with jigger/jolley.

If you had, say, 6 slip-casting moulds, at the best you could cast 6 plates per day.  With one jigger/jolley mould, you would be able to re-use the mould multiple times per day.

Slip-casting moulds have a finite life-span - around 30-40 casts (from memory) casts before the plaster starts to deteriorate.  The jigger/jolley moulds will allast much longer as they do not come into contact with an ingredient included in slip.

[Brooklyn Mom]

He’ll Rareearth! I too do slip casting as many others successful potters. I’m using Tony Hansen’s L3778G recipe https://insight-live.com/insight/share.php?z=tgsPMxNsAP

i plan to sell it in my functional tablewware and expect it to withstand hard use. I just adopted this recipe so i don’t have personal  confirmation to give you but I’m banking on it

[Jeff Longtin]

The main thing that makes a plate strong is that the clay has been compressed during the throwing process. That is why the jigger/jolley method is used to make commercial plates. (The jigger, the template, compresses the clay against the jolly, the plaster form.)  The process of slip casting does not compress the particles. (Instead, the clay particles "settle" into alignment.)

Using a porcelain slip cast body that becomes vitreous would increase its strength but it will also increase the chance it will deform in the firing.

[Brooklyn Mom]

Thanks Jeff! I’m looking into making/figuring out how to make my own jitter/jolly system. The learning curve has been so steep for me; from learning how to make plaster molds, making my own ceramic slip, my own underglazes and glazes, ...Now adding how to make my own plastic porcelain equivalent to the liquid and a jitter system  it’s been fun though and I’m glad to have the control. 

[Min]

2 hours ago, Sig Student said:

Now adding how to make my own plastic porcelain equivalent to the liquid

Same recipe minus the darvan + more plasticizer. If you use more VeeGum you will need to decrease the nep sy as the VeeGum will increase fluxing. Since veegum is so expensive you might want to try the whitest bentonite you can find instead or a mix of the two.

[Mark C.]

Porcelain is less prone to chipping vs stonewares in a general sense.

The compression factor cannot be overstated for helping make clay even stronger .

Slip cast can be strong but its very dependent on the form as well as the right body/firing temp.

You can knock out jiggered plates fast on molds and they are strong.-If you are a great thrower you can knock them out nearly a fast as well.

[Jeff Longtin]

Years ago a local supplier, Minnesota Clay, sold a clay body called Polar Porcelain. I know a few folks who bought the powder form and added deflocculant to turn it into a slip cast body. Not every clay body will adapt so easily but some do. 

[Brooklyn Mom]

On 4/1/2021 at 5:06 PM, Min said:

Same recipe minus the darvan + more plasticizer. If you use more VeeGum you will need to decrease the nep sy as the VeeGum will increase fluxing. Since veegum is so expensive you might want to try the whitest bentonite you can find instead or a mix of the two.

This is good to know Min.  Thank you