Sculpting & Modelling > Sculpting & Modelling Questions

Baking on glass items like jars and vases

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. . . and be careful about magnets.  Everything I have read, and a couple of teachers too, says that baking magnets causes them to lose significant strength.  What you do is you leave a magnet-sized cavity on your piece, bake it and glue the magnet in after.

You can also carve out a magnet-sized hole afterwards.

Great project! 

My problem with this is that the clay cracks and not the glass. Don't know what to do

Hi, you may be better starting another thread as this one is so old, it may get more responses.

But if the clay is cracking - are you conditioning it correctly/enough?  Are you baking at the correct temp for the brand you are using?

Or could it be you are putting clay over something that changes shape and size when baked such as wood?

At the temperatures clay is baked at, there should be no problems with glass, but as others have said, it would be best to put the piece in a cold oven and let it heat up with the oven temperature.  My oven doesn't take long to heat up - so I would just bake the clay for the usual length of time.

Pyrex glass is especially made for putting in ovens.  Because of the way glass expands during heating - one side expands quicker than the other, and that's why it cracks - especially if going from room temperature into a hot oven.  So pyrex glass could go in a hot oven, but normal glass should be heated up slowly and, as Emma said, allowed to cool down with the oven too - or at least handled very carefully if removed when hot as the slightest tap could crack it.  If glass begins cracked, the heat will exploit that weakness and it will crack further.

--- Quote from: Ceni85 on March 25, 2016, 05:11:17 PM ---My problem with this is that the clay cracks and not the glass. Don't know what to do

--- End quote ---

Clay probably expands at a different rate to glass - so if the glass inside expands more than the clay and there is nowhere for it to go, it will crack the clay.  Try doing what Emma suggests and leave a gap at the seam so the clay can move with the glass as the glass expands during heating.  Again, heating slowly will mean expansion will be more gradual and the risk of cracking will be reduced. 


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