Panda Bears with cracks following first firing.

[Terri98]

I generally Raku my animal sculptures.  My last batch of clay is giving me some challenges with cracking.  I have several, some bisque ware that presented with cracks and some greenware.  My clay has Kyanite but I am not sure it has enough grog.  It is a special blend.  Unfortunately it is presenting problems.  I have heard that paper clay can be used to fill cracks  and than I re-fire.  The clay I am currently using is not a paper clay.   I would still like to Raku if possible. 

Thanks Terri

[Babs]

19 hours ago, Terri98 said:

I generally Raku my animal sculptures.  My last batch of clay is giving me some challenges with cracking.  I have several, some bisque ware that presented with cracks and some greenware.  My clay has Kyanite but I am not sure it has enough grog.  It is a special blend.  Unfortunately it is presenting problems.  I have heard that paper clay can be used to fill cracks  and than I re-fire.  The clay I am currently using is not a paper clay.   I would still like to Raku if possible. 

Thanks Terri

You made these? Cracks  where arm is joined may be a drying, difference in moisture of pieces at time of joining. What slip are you using. May need to go for a deflocculated slip or "magic water". Solid legs , cracking where meeting body, again s moisture imbalance when drying. Might need to pierce the legs from feet up many times with skewers after making. Dry long and slow in a container. These have been bisqued? Can use a product "SPOOZE" or some folk use a commercial product. Need to rebisque

 

[Chilly]

When I'm teaching newbies, I always say, nothing thicker than my thumb.  I think thickness and uneveness is causing the cracks during drying.

If you imagine a sphere, say 150mm across, with a thickness of 10mm.  The clay on the outside will shrink and crust over as the moisture evaporates.  The crust makes it harder for the moisture deeper in to evaporate, and as no evaporation, it can't shrink.  So the outside is shrinking and the inside isn't.  Tension pulls something apart and you get cracks.

[neilestrick]

I also think it's a thickness issue. Make them half as thick, and hollow out the legs.

[Pres]

Definitely a thickness issue. Years ago, I had a student that made animal objects at home with our clay. They brought them in while still wet for me to fire them. looked at the pieces that were about 8 inches in length and 5 in width. Felt way too heavy. So I told them so and we decided on a plan to lighten them up without losing the form. On the legs, she cut long deep grooves and then used a child's spoon to hollow out the inside, then sealing it back up with a slab of clay the same thickness as the leg area, We followed this plan with all the body parts, and found ways to leave hidden air holes to help with drying and firing pressure. Drying was done over a two week period, and firing turned out well along with the final glaze firing. Years ago, but I remember it well . . . . especially the excited smiling face after seeing them being unloaded from the kiln.

 

best,

Pres

[Terri98]

Yes, the two Pandas are bisque-ware to cone 04.   My pieces are normally thick.  Hand built.  I would have to say most of my sculptures do not have consistent thickness in the walls.   I am aware it would be nice if that were possible.   Usually not a problem with the way I let them dry very very slowly and do a very slow bisque fire.  Than I Raku.  This is a special blend clay.  It has Kyanite.  Not sure it has enough grog.  I am hoping the clay maker made notes, so I can learn what he put together and what I might need to do to make the clay more user friendly. Whatever I do, I will have to knead it into the clay.   In this case the thickness is part of the issue. 

On these two pieces I hollowed out from the bottom.  I may have cut off and added back certain parts.  When I do that the slip I use is the same as the clay  with vinegar added.   Could have skewered hind legs from the inside.  But I did not.  I think I did hear or read about "SPOOZE" regarding repairs.  And also the need to re-bisque.  When I think of  deflocculated slip, it is something that is used to pour molds,  so I would not think it would work with what I do.  I don't think I have heard of  Magic Water.  I will have to look that up too.

My primary question, is how do I repair the cracks and save the Pandas.  I am hoping to find a repair solution using ceramic paper clay.  I hear people talk about this and want to learn more.  Do I need to change how I ask my question?

Thanks

Terri

 

Edited October 11, 2023 by Terri98
clarification

[Callie Beller Diesel]

Spooze and magic water are similar enough to each other, but I don’t know if either is the right tool for this job. Re: deflocculated slip, most white clay bodies can be deflocculated, and the results will create an incredibly strong join. 

I agree that making a patch slip with some paper fibre in it to fill non-structural cracks would be a better option in this case. Mixing some up is pretty straightforward if you’ve ever done paper making with a blender.

To make the pulp, use the really cheap toilet paper, 1 ply if you can get it. This is no place for the quilted stuff the cartoon bears like. Tear it into squares and soak it down in enough hot water to make it go through the blender easily. Process it until it’s well disintegrated, with no distinct pieces. Drain well and gently press out the excess water in a sieve, but don’t compress it. 

Make some slurry out of your clay body: if you have dried pieces you can slake down, this is ideal. If not, slice some existing clay into really thin pieces and slake in hot water for an hour or so, and a stick blender should be able to handle making about a cup of slip that’s a thick pudding consistency. Add no more than 30% by volume of the paper pulp to the slip and mix well with the stick blender. You can eyeball this: it doesn’t need to be exact. It’s just one of those “some is good but more is not better” sort of guidelines. 

You can either use the slip as is to fill small cracks, or dry it out into a more typical working consistency to fill larger ones. The paper fibre will help fresh clay bond to both dry clay and bisque. The join may be somewhat visible due to the smear marks from blending the clay: this isn’t super avoidable. If you feel really strongly about it, sanding some of the texture down after the piece is bisqued can help. You will have to re-fire any patches on bisque before glazing. 

 

[Babs]

Reason I suggested deflocculated slip is at the same consistency, it will contain much less water and so less shrinkage as it dries, and thus less cracking, also what Callie wrote above re strength of bond.

Any mending will have to really dry out before refiring or going straight to glazing, I'd try one panda first.

Beautiful pandas btw.

[neilestrick]

On 10/10/2023 at 8:24 PM, Terri98 said:

I am hoping the clay maker made notes, so I can learn what he put together and what I might need to do to make the clay more user friendly.

Wedging grog or other particulates into the clay will certainly help, but working that thick really needs an adjustment to the firing and cooling schedule in the kiln, too. And grog may not allow you to work the way you're used to or allow for certain surfaces you want. The simplest solution is to just work thinner. Get in there and carve out the thick areas.

Magic water is just the stuff you add to clay to make deflocculated slip, and deflocculated slip makes for a very strong joint. I don't think the cracks you've got here are a result of poor joining, though.

[Hyn Patty]

As an animal sculptor myself, your pieces are beautiful.  Lovely work! 

I agree using a bisque mender (I don't bother adding paper fiber myself) will work great and you can carefully tint it with underglaze or clay to match your original clay.  If the color after firing is off just a touch and bothers you, you can softly apply a little opaque underglaze to the area to blend it in and re-bisque one last time to hide it.  Again matching your colors to your original unglazed piece so it's not visually a problem.   I repair bisques of various kinds this way all of the time, even thin broken legs.  If you are able to rough up the edges of the break the mend will be stronger than just filling in a smooth crack.

If it is at all possible to deal with your thickness issue you should.   Even with hand building you can always start with a wadded up bit of paper as your core for pieces that will be too small to get your fingers into to hollow out properly.  Dry it slowly and the paper will burn out in the first firing.   

May even want to make plaster molds of various parts before assembly so you can slip cast replicas that then you can resculpt anew into individual pieces and save you both some time and cracking, but up  to you.  Good luck with it!

[oldlady]

terri, why do you raku your beautiful pandas?   do you see an advantage i cannot think of?