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preeta

Clay Thickness - Greenware Vs Bisqueware

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I am trying to figure out how to ask this question intelligibly.

 

does the thickness - the walls of clay get thicker when the greenware is fired as bisqueware?

 

I over-trimmed a large bowl the other day and instead of chucking it out i decided to fire it to see what would happen.  One of the walls (i hadnt centered perfectly to trim, plus i was impatient and it was on the wet side) was super thin compared to the rest of the bowl. so thin that it was buckling but since the rim was drier it did not collapse.

 

then when it came out of the bisque load i could not feel that 'chip' quality.

 

which started a whole lot of questions in my head. 

 

we are asked to keep about 1/4" at the bottom while throwing. we are told we dont want a thick bottom. but then we have to trim. i am assuming as newbies who compress the bottoms we might add more clay there.  Dont we trim the foot and take off even more clay? does that mean the bottom becomes really thin too like 1/8 of an inch? we are asked to not make our walls too thin. 

 

i have overtrimmed feet before where the circumference just around the feet is so thin that i have to dry the bowl on the lip. if i put it back on the foot it would collapse where i had gone super super thin. (i used that bowl as a glaze test and also because it was the best bowl i had thrown so far then and it was too painful to throw it away - you can see the line and feel it too where i had trimmed it too thin). we use that medium bowl very frequently for the last year. nothing has happened to it yet. 

 

i am asking this question here because i cant test my theory for another two months till school starts again.

 

but logically my conjecture should not add up because clay shrinks in firing. i would imagine it would shrink further that its greenware  wall thickness. 

 

i hope i make sense. 

 

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Clay shrinks -it does not expand (unless its a refired pot-and that is only when heating)

If your wall was to thin and you bisqued it it is still to thin.

One thing you may  not have thought of is your piece was seemly to thin but maybe to wet and was collapsing not to thin-now bisqued its harder.

Just a thought. When I;m impatient and trim to wet its very hard to feel whether the work is to thin or to thick as its water weight screws me up with thickness feeling. Hence I try to not trim when pots are to wet.

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Actually the clay expands during a bisque fire, up to 2050F: then it starts compacting. The most expansion I have seen in studies done was around 3.5%: not enough to account for what you are describing. Mark,s explanation seems to be the most likely cause.

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It's not a usual thing for a beginner to add more clay to the bottom of a piece during compression: usually it's the opposite, and you wind up with a divot in the middle.

 

You ask if things can thicken in the bisque: no, but they can change somewhat in proportion in a way that highlights the fact that you may not have trimmed enough off, or you think maybe you overshot the mark but actually found out that your clay has more ability to stand up than you thought.

 

If you're trimming the right amount off a piece, the cross section of the bottom should be equal to that of the walls of the pot, whatever that thickness is. Ideally, yes, it should be in the 1/8" neighbourhood, depending on your clay and skills. I aim to have a trimmed piece that feels light for its size in the bone dry state.

 

If you've left too much weight in the walls, it is ok to trim that down, too, and aim for better tnrowing skills on the next round. If you leave a pot uneven, it leads to all those problems that cause cracking, from uneven drying to uneven heating, to glaze tension issues (in extreme cases).

 

Learning to judge the right thickness is part of the learning curve, and is best mastered through everyone's favourite pastime ...practice.

 

I find it helpful to take a moment before I centre a piece to trim and "feel out" where the excess is that I need to remove to achieve that even cross section.

 

we use that medium bowl very frequently for the last year. nothing has happened to it yet. 

 

 

When it does eventually break (if it's that uneven, it's only a matter of time: it will break if you're using it regularly) or when you can feel confident enough in your abilities to break it on purpose, study the cross section and use it to understand what you did.

Min likes this

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aaah. this means i need to pay attention to thickness next. so that will be my next focus.

 

mark yes i have noticed. it was difficult to gauge the thickness of the clay that i dug out. but i could tell it was thinner than the rest. i could not really tell how thin. however yes i am starting to pay attention to the thickness of clay.

 

i am starting to get a feel for what the weight of the ware should be. but i still dont have enough 'feel' on thickness. 

 

i am starting to play with thickness. the focus in school is even and thick (i'd imagine its easier to do even with thicker than thin) and i am the only person who pushes it. 

 

i've just gotten to understand centering through feel. it is just starting to make sense. 

 

i have just played with thickness but not given it full thought.  its time to do that now. 

 

callie the old bowl i gouged out is kinda heavy. i'd say the walls are about 1/4" thick with the gouged out area being probably 1/8th and there was more clay at the bottom.  its a bowl whose proportion is perfect, the glaze is perfect, and its shape is one of the best i have made. and therefore its hard for me to break it. but there's a valley where i took out the extra clay. heavy handedness.  its a line in glaze that is very easily noticable. 

 

now  i need to cut through trimmed pieces to see what i think and what is really true. 

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Make an extra two pieces each day if your wanting to keep your work to test/practice glazes. The first piece of the day you make set aside label it as 1. Then before you finish for the day make another of the same shape. Label it 2. Then when you go to trim. Trim that first piece and cut it in half. Look at your work. Trim the rest of your work from that day. Then your last piece you made labeled 2 trim it and cut in half. You will see the difference even a little bit of practice makes and continue this til you are happy.

Min, douglas and oldlady like this

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"we are asked to keep about 1/4" at the bottom while throwing. we are told we dont want a thick bottom. but then we have to trim. i am assuming as newbies who compress the bottoms we might add more clay there.  Dont we trim the foot and take off even more clay? does that mean the bottom becomes really thin too like 1/8 of an inch? we are asked to not make our walls too thin." 

 

1/4" isn't enough if you want to trim a foot, need's to be about 1/2". If you are aiming for 1/4" walls then the base should be 1/4 also inside the footring. It  sounds like your instructor is aiming for even walls/base a bit thick then working towards making them thinner once you get the eveness sorted. 

 

When you are trimming tap to hear when the sound changes from a thunk to a lighter tone, also press gently on the base, it will have a slight give to it when thin enough (as long as the pot isn't overly dry). If you trim the base first then imagine a smooth line from the base through to the sidewall, if the base is trimmed properly then the walls will have a better chance of being thin enough. One continuous line from base through sides if you followed the curve under the footring. Make sense? When you pick the pot up it should't feel bottom heavy after trimming.

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The goal should be 1/4†thickness after trimming...i tend to like a fancier trimmed foot on a bowl so i tend to leave more clay so i have more clay to play with, my cylinders on the other hand get the minimum amount of trimming so a almost a 3/8†bottom gets thrown so i end up with hardly any trimming With a shallow footring.

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preeta, i am glad you have conquered centering.  Mea was here on sunday and i confessed to having a problem missing the exact center at times.  she showed me the best secret i have ever seen.

 

if you touch the outside of your centered clay and allow your finger to spiral toward the center, your last touch will be in the exact center.  MIND-BLOWING!! :D

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