Brown Slip warping and cracking?

[Bear]

Hello, I have a question about a certain slip casting recipe. (Quick Backstory) I work at a ceramic factory that forms with slip in plaster molds. We have two clay bodies for slip casting, one of which is a dream to work with. However the other seems to be a bit more problematic. The pieces have a high chance to warp in the kiln once fired to cone 7. They also seems extremely happy to crack in the mold on the handles of mugs and T cups. I also have notices once the pieces are finished and cleaned with water and a sponge, small longs cracks inside the mug start to appear once dried. This is not all mugs but some.  I have not gotten the proper knowledge behind recipe formulation to review the recipe and conclude a certain error in its ingredients or proportions of ingredients. I would like to just post the recipe and ask if their are any glaring issues that pop out? 

 

54.8041237113402 %                  REDART

13.855670103092784%             ICEBERG/KAOLIN

10.88659793814433%                SILICA

9.896907216494846%                KYANITE

5.938144329896907                    HTP

3.958762886597938%               SPODUMENE

1.0309278350515463%            BENTONITE

        My apologies for the format, Thanks!

 

 

 

 

[liambesaw]

It looks like it's mostly redart as the clay body and redart has really low plasticity, I don't see any Ball clay or anything else to increase plasticity but I also don't know what HTP is.  I don't have any experience slipcasting but plasticity seems like it would be pretty important.

[Mark C.]

What country are you in?

whats HTP?

also what is Iceberg kaolin's alumina content-if you had say epk (Edgars Plastic EPF from Florida it has enough alumina to keep warping down-you could just as say 3% more but since I have never heard of Iceberg epk I do know the contents of it?

When our cone 10 slip cast products slumped we cured this by a cheap fix of just little more alumina which was in the epk.

[Bear]

I will have to get some more specifics tomorrow, and i am in America

[Min]

HTP is a Tennessee ball clay, looks like Iceberg has higher alumina and silica than EPK, particle size is about the same.

[glazenerd]

Bear:

several things wrong with this recipe, from a clay chemistry stand point. The first being: clay recipes do not require that type of precision in weight measurements: that appears to be a carry over from glaze calculation. There is zero difference in outcome if you use 55% Redart in lieu of 54.8014..%.  When using  plasticizers such as macaloid or V-Gum T: 1/2% additions become necessary because of the properties of these types of materials.

this recipe is depending upon kyanite to produce mullite, in lieu of the typical glass melt feldspars produce. Kyanite converts to mullite when you get into the cone 5-6 range: and is thought to produce better strength because of the elongated needles that form. Typically 3-5% is added for additional strength, but it is not used as a primary glass former.

spodumene supplies lithia: which is used to control thermal expansion properties of a clay body. Oven ware bodies, with very low COE index, use spodumene almost exclusively. Sodium and potassium for example have individual expansion indexes ranging from 90-100 ( round figures), and spodumene is roughly 1/3 of that. However, it takes a fair amount of spodumene to supply enough lithia to form any kind of glass phase. 

Bentonite is probably being used in a two-fold purpose: as a plasticizer and to supply extra fine particle distribution. More likely than not, this is sodium bentonite which is subject to swelling and high water absorption. (Absorbs up to 15 times it's weight in water.) used as a suspension agent in glazes, but does not supply the same effect in clay/slip recipes. If you are going to use it: better to blunge it in a slurry before adding to the slip. Dry additions do not distribute very well in clay bodies.

As a general rule of thumb: kaolins supply 37% alumina and ball clays 25-30%. Clay formulation typically aims for a 4:1 SiAl ratio. Unlike clay bodies formulated for throwing: slips should be on the low to medium plasticity level. You have other issues going on as well, but will let you update the info.

 

t

[glazenerd]

Bear:

your recipe in whole numbers: Redart 55%, kaolin 14%, silica 11%, kyanite 10%, HTP 6%, spodumene 4%, bentonite 1%. 

Analysis: total KnaO 2.71, total flux 4.58%, iron 1.86%, silica 76.15%, alumina 16.77%, SiAl ratio 4.54 est. COE 5.45

2.71% molar KnaO (sodium & potassium) too low for cone 7. This recipe is depending upon the natural fluxes found in the Red Art, kaolin, and HTP. Bentonite is not required in this recipe because the HTP has a high CEC ( plasticity) and finer particle size. Kyanite forms elongated mullite needles that add strength, but they do not supply the structured glass phase required. 4% spodumene I assume was added to lower expansion, it does not supply enough flux to promote a melt. From the analysis,  this is a modified cone 10 recipe. I believe the warping stems from the kyanite and cracking from the  lack of glass phase. 

Redart 50%, kaolin 20%, silica 6%, kyanite 5%, HTP 8%, spodumene 4%,  bentonite 0, Kona F4 7%.

Anaylysis: KnaO 3.40%, total flux 5.29%, iron 1.74%, silica 74.96%, alumina 17.34%, SiAl 4.32, est. COE 5.69

When formulating stoneware recipes, rather throwing or slips: one principle has to be observed. There has to be enough KnaO available to melt the silica. Typically stoneware recipes have a minimum 10% KnaO additions to ensure that silica Is incorporated into the melt. Any free silica converts to cristabolite, which destabilizes the structural strength. Have you heard pinging come from the kiln in the 400F range?  That would be cristabolite inversion. It can cause the micro fissures you describe as well. I tried to keep this in line with your original recipe, but personally I would go 10% Kona or equal, and reduce the silica to 3%. Reducing the silica into the 72% range would not hurt.

tom

slips have differing particle packing densities than clay bodies. How you mix, how quickly you pour afterward, deflocculants all play a role in collidial chemistry.