Livia Posted March 17, 2020 Report Share Posted March 17, 2020 Hello! I'm new at casting ceramics and I have some problems that I can't troubleshoot on my own. I get some spots on the bottom of the cast pieces after the second firing (1220 C). They don't seem to be connected with the mould because the spots are different on every bowl an plate. Also, on the unglazed surface I get this brown mud-like marks that seem to be coming from sponging the piece but the sponge is perfectly clean and the casting mold was new so no other clay touched this clay. Do you have any idea why this 2 things happen? For some reason I can't insert the picture but below is a link to the photo in my dropbox: https://www.dropbox.com/s/hul9yxeg3d8bdos/IMG_20200317_160450.jpg?dl=0 Thank you for your help! Link to comment Share on other sites More sharing options...
Livia Posted March 18, 2020 Author Report Share Posted March 18, 2020 5 hours ago, Rockhopper said: What clay/slip are you using ? Any chance you unintentionally picked up a speckled clay ? Hi! Yes, I was using a G&S 933 clay with basalt speckles. I made my own slip as this ceramic body is not available for casting. Do the spots and the muddy appearance have any connection with the speckles in the clay body? Is there a way to prevent this? Thank you! Link to comment Share on other sites More sharing options...
Livia Posted March 18, 2020 Author Report Share Posted March 18, 2020 @Rockhopper I like the speckled effect, that is why I have bought this clay and definitelly don't want to take them out. But the marks/spots I'm talking about and the muddy appearence is something else, maybe something made by a gas during the second firing. Yeey, I've managed to upload the photo...See the bottom of the bowl. Link to comment Share on other sites More sharing options...
Livia Posted March 18, 2020 Author Report Share Posted March 18, 2020 Thank you! Yes, just the area on the right side, the dark spots down on the left are the basalt spots from the clay. Never used this type of clay before, however I have used 3 molds - 2 of them were new, one was previously used and never had the same problem with other clay. The mark on the bottom appear on every single piece made with this clay, after the second glaze firing and not in the same place. The kiln was not tightly packed, the pots did not touch eachother in the kiln. I bisqued with 60 degree C/hour up to 600 C than 120 C / hour up to 1020 with a soak of 20 minutes at the end (cone 06, I think) Second firing was at 80 degree C/hour up to 600, than 125 up to 1220 (cone 5, I think) The air vent was halfway open the whole time. Link to comment Share on other sites More sharing options...
Min Posted March 18, 2020 Report Share Posted March 18, 2020 11 hours ago, Livia said: I made my own slip as this ceramic body is not available for casting. What is the process you use to mix this body into a casting slip? Link to comment Share on other sites More sharing options...
Livia Posted March 18, 2020 Author Report Share Posted March 18, 2020 added water, mixed very well, let it sit for 2 days i also wanted to add a deflocculant at some point but the mixture seemed fine without it so I didn't Link to comment Share on other sites More sharing options...
Livia Posted March 18, 2020 Author Report Share Posted March 18, 2020 2 minutes ago, Rockhopper said: A complete guess here, as I've done very little slip-casting: Is it possible that the dark area is occurring in the spot where the slip first hits the bottom of the mold as you pour it in ? (Don't know why it would cause that mark, but it's the first thing that came to mind that might explain why you're getting similar marks, with different molds, but all on the bottom.) It's possible that a higher bisque fire would clear it. According to my Orton chart, your top temp of 1020 C would put you around ^05-1/2. Many potters bisque at ^04 (around 1070 C) If everything else seems fine, I can definitelly try to bisque higher. Thanks! Link to comment Share on other sites More sharing options...
Min Posted March 18, 2020 Report Share Posted March 18, 2020 30 minutes ago, Livia said: added water, mixed very well, let it sit for 2 days i also wanted to add a deflocculant at some point but the mixture seemed fine without it so I didn't Okay, I think this is the problem. Try a batch using deflocculant. If you need them basic directions for mixing a casting slip here. Far too much water in a slip made without a deflocculant for it to cast properly. Link to comment Share on other sites More sharing options...
Livia Posted March 18, 2020 Author Report Share Posted March 18, 2020 1 hour ago, Min said: Okay, I think this is the problem. Try a batch using deflocculant. If you need them basic directions for mixing a casting slip here. Far too much water in a slip made without a deflocculant for it to cast properly. Thanks! Will look into it! Link to comment Share on other sites More sharing options...
Babs Posted March 18, 2020 Report Share Posted March 18, 2020 Could it be localised reduction... Is it obvious prior to firing? Link to comment Share on other sites More sharing options...
Livia Posted March 19, 2020 Author Report Share Posted March 19, 2020 18 hours ago, Babs said: Could it be localised reduction... Is it obvious prior to firing? No, it was nothing there. Does this mean that because of the bowl's foot the gasses stayed under the bowl? If it is so, is there a way to prevent this with the help of the kiln ventilation? Link to comment Share on other sites More sharing options...
Magnolia Mud Research Posted March 19, 2020 Report Share Posted March 19, 2020 12 minutes ago, Livia said: is there a way to prevent this with the help of the kiln ventilation? set the items of little wads of clay or wadding to allow some ventilation to the interior of the foot rim area. LT Link to comment Share on other sites More sharing options...
Livia Posted March 19, 2020 Author Report Share Posted March 19, 2020 1 minute ago, Magnolia Mud Research said: set the items of little wads of clay or wadding to allow some ventilation to the interior of the foot rim area. LT Thanks, will do this! Link to comment Share on other sites More sharing options...
Babs Posted March 19, 2020 Report Share Posted March 19, 2020 6 hours ago, Livia said: No, it was nothing there. Does this mean that because of the bowl's foot the gasses stayed under the bowl? If it is so, is there a way to prevent this with the help of the kiln ventilation? Area being starved of oxygen because closely packed or in your case, foot ring stopping air flow..maybe Link to comment Share on other sites More sharing options...
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