Hand-building advice

[Gabby]

When you hand-build something by putting together two identical pieces done separately, like two hemispheres to make a sphere, how dry should the two halves be before you fix them to each other? Leather hard? Somewhat less that leather hard?

I suppose this would be the same in wheel throwing. I have read that Korean moon jars are made from attaching two halves thrown separately on the wheel and fixed at the equator.

Is the answer different for assembling something you can still reach into (unlike a sphere), like something with on oval footprint with front and back cut separately?

I feel like I tend to put things together when the clay is still too soft to hold its shape very well, with funky results.

[LeeU]

I mostly join when the elements are about "cheese hard", or a bit shy of leather hard. However, if I want a particulary geometric,  or structured appearance, I do wait for leather hard as that lends itself to a bit more precision.

 

[Callie Beller Diesel]

This is definitely one of those "it depends" questions.

I was trained by people that would attach freshly thrown elements to each other to make thrown and altered pieces. It takes a sure touch and a lot of practice, but that method has the benefit of not needing any slipping or scoring to adhere.  You need to go back in when things set up and compress things a little, and to sometimes reinforce some joints that may be prone to cracking. It can be a very immediate way of working, but you do really need to know how your particular clay body is inclined to warp.

I have also witnessed people successfully attach pieces at the point of leather hard where there's no flex at all left in the clay. This method has a lot more slipping and scoring involved, and it works a lot better with stoneware than with porcelain.

My clay tends to be prone to easy rehydrating, so it's reasonably forgiving at what point you attach things. As long as it has a little bit of time covered so that the moisture in the pieces can equalize with each other before drying, it works. It can have a pronounced plastic memory, so having the mug bodies firm enough that they don't distort when I push the handle on is a good idea.  

My best, and unfortunately kind of annoying suggestion is to maybe push a couple of test pieces to find the point at which your clay "breaks," so to speak.

[Gabby]

8 hours ago, Callie Beller Diesel said:

This is definitely one of those "it depends" questions.

I was trained by people that would attach freshly thrown elements to each other to make thrown and altered pieces. It takes a sure touch and a lot of practice, but that method has the benefit of not needing any slipping or scoring to adhere.  You need to go back in when things set up and compress things a little, and to sometimes reinforce some joints that may be prone to cracking. It can be a very immediate way of working, but you do really need to know how your particular clay body is inclined to warp.

I have also witnessed people successfully attach pieces at the point of leather hard where there's no flex at all left in the clay. This method has a lot more slipping and scoring involved, and it works a lot better with stoneware than with porcelain.

My clay tends to be prone to easy rehydrating, so it's reasonably forgiving at what point you attach things. As long as it has a little bit of time covered so that the moisture in the pieces can equalize with each other before drying, it works. It can have a pronounced plastic memory, so having the mug bodies firm enough that they don't distort when I push the handle on is a good idea.  

My best, and unfortunately kind of annoying suggestion is to maybe push a couple of test pieces to find the point at which your clay "breaks," so to speak.

Please explain further how the state of the clay at point of assembly is related to warping.

[1515art]

Gabby, depends on what I’m making and just how I’m planning on finishing, trimming and those things. For larger pieces when I’m working wet so I can touch, shape and handle without the shape getting damaged I’ll use a heat gun to firm up the clay in critical places and leave things softer at the joints/join areas and where I’m planning other shaping or  more throwing, using a soft touch is also important. When working with wetter clay I can work the seam making everything one piece. Same thing throwing a tall cylinder sometimes  I do it in two sections throwing the bottom section normally, the top section is the same diameter thrown bottomless. Then just a tiny bit of firming with the heat gun before quickly inverting the second cylinder and securing It onto the top of the first cylinder when the pieces are joined air trapped inside helps support it structurally until the top batt is wired off from what is now the top of the tall cylinder. If the clay is still to wet to support its own weight use the heat gun to firm it up before wiring off the batt.  An additional advantage inverting the top section is any clay thickness in what is now the top of the cylinder can be thrown more if everything is still on center. I know one artist that makes very large round forms joining slabs of clay around different size beach balls, deflating the ball and removing it when the clay was firm enough to support its own weight.

Sometimes leather hard is best for structural reasons and then greater care is required reinforcing joints, fitting parts and controlling the drying in order to avoid the clay warping and seams opening/cracking and pieces falling off.

 If pieces do fall off that’s ok too, joining bone dry pieces is often the trick. In China at the factories I’ve watched the make many different forms in large sections and join the sections dry with a clay mortar and trim  for final shape after assembly. The challenge working with the clay dry is obviously it needs to be correct size and shape as dry clay bends poorly. The advantage joining pieces when everything is dry are no stresses due to uneven moisture content in the clay, usually I mix up some thick slip from the same clay and a bit of paper fiber in a blender to the consistency I need to glue things together. The dry pieces cement quickly and the thick slip can be sculpted a little to fill gaps and transitions.

 

[Gabby]

4 minutes ago, 1515art said:

Gabby, depends on what I’m making and just how I’m planning on finishing, trimming and those things. For larger pieces when I’m working wet so I can touch, shape and handle without the shape getting damaged I’ll use a heat gun to firm up the clay in critical places and leave things softer at the joints/join areas and where I’m planning other shaping or  more throwing, using a soft touch is also important. When working with wetter clay I can work the seam making everything one piece. Same thing throwing a tall cylinder sometimes  I do it in two sections throwing the bottom section normally, the top section is the same diameter thrown bottomless. Then just a tiny bit of firming with the heat gun before quickly inverting the second cylinder and securing It onto the top of the first cylinder when the pieces are joined air trapped inside helps support it structurally until the top batt is wired off from what is now the top of the tall cylinder. If the clay is still to wet to support its own weight use the heat gun to firm it up before wiring off the batt.  An additional advantage inverting the top section is any clay thickness in what is now the top of the cylinder can be thrown more if everything is still on center. I know one artist that makes very large round forms joining slabs of clay around different size beach balls, deflating the ball and removing it when the clay was firm enough to support its own weight.

Sometimes leather hard is best for structural reasons and then greater care is required reinforcing joints, fitting parts and controlling the drying in order to avoid the clay warping and seams opening/cracking and pieces falling off.

 If pieces do fall off that’s ok too, joining bone dry pieces is often the trick. In China at the factories I’ve watched the make many different forms in large sections and join the sections dry with a clay mortar and trim  for final shape after assembly. The challenge working with the clay dry is obviously it needs to be correct size and shape as dry clay bends poorly. The advantage joining pieces when everything is dry are no stresses due to uneven moisture content in the clay, usually I mix up some thick slip from the same clay and a bit of paper fiber in a blender to the consistency I need to glue things together. The dry pieces cement quickly and the thick slip can be sculpted a little to fill gaps and transitions.

 

Thank you for your detailed response. I will read it many times.

[Dick White]

It is my understanding and experience that clay warps for 2 reasons - 1) Uneven drying results in the clay shrinking more on the drier side, putting tension on the still-moist side that has not yet shrunk as much. It may seem to straighten out, but it comes back in the firing. 2) Stretching and bending during the process of making the piece may seem to be put back in place, but it returns in the firing.  With wheel thrown work, an errant hand movement may cause a bulge in the cylinder that is difficult to repair. For thrown and altered work, the warping is intentional. With handbuilt slabs, even  gently "peeling " it up off the table to move it will put a slight bend in the slab. Moist slab pieces are still flexible and will bulge and distort slightly during assembly. You can smooth it back into place, but the warp will return. Conversely, if the slab is moved by flipping from one flat surface to another, and the cut pieces are allowed to firm up before assembly, the pieces can be handled without bending and any intentional curvature can be added once without the back and forth of trying to put an undesired bulge back where it belongs.

[Gabby]

12 minutes ago, Dick White said:

It is my understanding and experience that clay warps for 2 reasons - 1) Uneven drying results in the clay shrinking more on the drier side, putting tension on the still-moist side that has not yet shrunk as much. It may seem to straighten out, but it comes back in the firing. 2) Stretching and bending during the process of making the piece may seem to be put back in place, but it returns in the firing.  With wheel thrown work, an errant hand movement may cause a bulge in the cylinder that is difficult to repair. For thrown and altered work, the warping is intentional. With handbuilt slabs, even  gently "peeling " it up off the table to move it will put a slight bend in the slab. Moist slab pieces are still flexible and will bulge and distort slightly during assembly. You can smooth it back into place, but the warp will return. Conversely, if the slab is moved by flipping from one flat surface to another, and the cut pieces are allowed to firm up before assembly, the pieces can be handled without bending and any intentional curvature can be added once without the back and forth of trying to put an undesired bulge back where it belongs.

This is very useful. Thank you.

[Callie Beller Diesel]

Thank you, Dick. You articulated that much better than I would have. 

[irenepots]

Excellent posts. Thanks