Strelnikov Posted August 20, 2013 Report Share Posted August 20, 2013 I have a mold in which as the slip dries it cracks. What causes this and what can be done to prevent it? I'm using slip straight from the shop, no used stuff added. Link to comment Share on other sites More sharing options...
BeckyH Posted August 20, 2013 Report Share Posted August 20, 2013 When does it crack? What shape is the object you are casting? Long, narrow stems will crack where they attach to the bowl of a goblet. Plates and bowls tend to crack across the bottom. A few possible reasons: The differences in thickness of different parts of the object are great enough that they dry at very different rates. You are dumping the extra slip before the casting is thick enough. You are letting your casting get too dry in the mold. You should pull it out when it is leather hard. Do you use a defloculant? If your slip is sludgy instead of smooth and creamy the differences in viscosity can cause problems. Link to comment Share on other sites More sharing options...
Strelnikov Posted August 21, 2013 Author Report Share Posted August 21, 2013 It cracks as it dries before it is pulled out of the mold. It is a bowl that is raised in the center. I think the problem is that the corners around the center part are not smooth enough. Is it possible to sand a mold to change its shape a bit? Link to comment Share on other sites More sharing options...
stephsteph Posted August 21, 2013 Report Share Posted August 21, 2013 i usually do not let my pieces dry all the way in the mold, and release them as soon as possible. it may be that as the piece is shrinking it is getting caught, either in a change in the relief or profile in the piece , or in a gouge or something. Link to comment Share on other sites More sharing options...
Mart Posted August 22, 2013 Report Share Posted August 22, 2013 It cracks as it dries before it is pulled out of the mold. It is a bowl that is raised in the center. I think the problem is that the corners around the center part are not smooth enough. Is it possible to sand a mold to change its shape a bit? Yes it is, it's plaster. Be really careful and visualize the effect the change has, before you start with alterations. Link to comment Share on other sites More sharing options...
PeterH Posted August 22, 2013 Report Share Posted August 22, 2013 Streinkov It cracks as it dries before it is pulled out of the mold. It is a bowl that is raised in the center. Assuming that you are using a 1-part drop-out mold and your de-molding problem is only marginal: Try to drop the cast from the mold repeatedly as it dries, so you de-mold as soon as you can. Alternatively support the upside-down mold just above a surface, so the bowl has a chance of dropping out itself.(Maybe on a board, and dropping the board an inch or so from time to time to encourage de-molding.) Otherwise, it sounds like the slip is shrinking onto the mold as it dries (and shrinks slightly). Do you have any reason to doubt this? Do you really have to pull the bowl from the mold, if so how strong and localised is the adhesion. There are three ways that you can address a shrink-on problem: remove the offending area (or reduce its severity "sufficiently"); de-cast very early (but the released form may not be self-supporting); use a parting agent[*] (and risk a slight surface blemish). You may need to use them in combination. When I've had the problem it was localised to one "face" of a multi-part mold: - I tried to de-mould this face as soon as the casting was strong enough to allow this without tearing, but without success. By the time the casting was strong enough to pull/lever off it was firmly stuck. - Using a parting agent made de-molding possible, but the freed face was very floppy (distorting ~1cm to de-mold). - I had to firmly support this face to restore its shape, and support it while it dried to leather-hard. Luckily it was a shape that permitted this (a flat face with an egg-shaped depression) and the other parts of the mold didn't get in the way. It rather sounds like you are using a 1-part drop-out mold. Which makes supporting a floppy de-molded bowl difficult. Regards, Peter * IIRC talc for lo-fire and nepheline syenite for hi-fire. 'ware dust hazards, and maybe make a dusting-bag from old tights (or even a tea-bag). Link to comment Share on other sites More sharing options...
Strelnikov Posted August 23, 2013 Author Report Share Posted August 23, 2013 Peter, Yes it's a 1-piece drop-out mold that I made myself. The cracking doesn't start appearing until the piece is almost ready to release. I think I'm going to try to modify the mold a bit to see if the cracking problem goes away. I made two other 1-piece drop-out molds that are working fine. Thanks very much for your help. Evan Link to comment Share on other sites More sharing options...
Strelnikov Posted August 23, 2013 Author Report Share Posted August 23, 2013 i usually do not let my pieces dry all the way in the mold, and release them as soon as possible. it may be that as the piece is shrinking it is getting caught, either in a change in the relief or profile in the piece , or in a gouge or something. It doesn't seem to be getting caught on anything. It is shrinking away from the mold, it just starts cracking as it shrinks. Link to comment Share on other sites More sharing options...
Strelnikov Posted August 23, 2013 Author Report Share Posted August 23, 2013 It cracks as it dries before it is pulled out of the mold. It is a bowl that is raised in the center. I think the problem is that the corners around the center part are not smooth enough. Is it possible to sand a mold to change its shape a bit? Yes it is, it's plaster. Be really careful and visualize the effect the change has, before you start with alterations. Okay thanks very much. Link to comment Share on other sites More sharing options...
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