Diane Pontoriero Posted November 16, 2019 Report Share Posted November 16, 2019 I fired 20 4"x4" raku tiles today and all but 4 broke either during the firing or as soon as they were removed from the kiln. I did fire them propped vertically on ceramic brick so I could remove them with tongs. They were hand built tiles with leaf impressions using Standard Ceramics 239 Raku clay. They cracked in half or close to in half. What caused them to crack? (I have been doing raku firings for 40 years). Diane Link to comment Share on other sites More sharing options...
Chilly Posted November 22, 2019 Report Share Posted November 22, 2019 Lots of questions come to mind: How thick were they? Were they bisqued? And dry? What was the temperature rise rate? Link to comment Share on other sites More sharing options...
Bill Kielb Posted November 22, 2019 Report Share Posted November 22, 2019 A few pictures might help with some speculation. @Chilly has all good points. How long did it take to go from room temp to 1800 degrees or so when you pulled them? 30 minutes means about 3600 degrees per hour! Link to comment Share on other sites More sharing options...
Diane Pontoriero Posted November 24, 2019 Author Report Share Posted November 24, 2019 The tiles were 1/4" thick and roughly 4"x4" square. They were bisque fired to cone04. We had been raku firing for about 3 hours when the tiles were ready to go in the kiln. I warm them on the top of my Olympic Raku Kiln before putting them in. Then slowly increase the temp in the kiln. By that time the pieces were maturing in 20-30 minutes. I use Standard Ceramics 239 raku clay. I am thinking that by resting them vertically on the hot fire brick caused stress and they cracked. May have to fire them flat and figure out how to pick them up. Diane Link to comment Share on other sites More sharing options...
Bill Kielb Posted November 24, 2019 Report Share Posted November 24, 2019 3 hours ago, Diane Pontoriero said: The tiles were 1/4" thick and roughly 4"x4" square. They were bisque fired to cone04. We had been raku firing for about 3 hours when the tiles were ready to go in the kiln. I warm them on the top of my Olympic Raku Kiln before putting them in. Then slowly increase the temp in the kiln. By that time the pieces were maturing in 20-30 minutes. I use Standard Ceramics 239 raku clay. I am thinking that by resting them vertically on the hot fire brick caused stress and they cracked. May have to fire them flat and figure out how to pick them up. Diane Done in 20 to 30 minutes is 3600 - 5400 degrees per hour. You might want to try slowing it down a bit and see if you can still leave them vertically stacked. Even with normal wares if I make temp in 20 minutes some things seem to crack more than if I slow things up a bit. Might be worth a try for the tiles. Link to comment Share on other sites More sharing options...
Dick White Posted November 24, 2019 Report Share Posted November 24, 2019 When you say "resting ... on the fire brick," are you using soft IFB brick or hard brick as the support? I used to put a cool shard of broken kiln shelf (of which I have a whole bucket, just for this purpose) on the hot shelf before loading each piece of ware on it to alleviate the initial thermal shock. Then I "discovered" setting an IFB brick on the shelf and loading the ware on it - don't have to swap out the brick for each new piece despite it looking like it is red hot. When I have a leaning piece such as yours, I set another IFB brick on edge to lean against. Now most of my breakage is due to poor construction of the joints, not body. Link to comment Share on other sites More sharing options...
Diane Pontoriero Posted November 25, 2019 Author Report Share Posted November 25, 2019 Yes, I am using soft IFB brick. I will definitely try your suggestion and slow down the firing. I will also have my student have her tiles ready for the first couple of loads so I can heat them up slowly. Thank you. I will report on our next firing. I agree up until these tiles, the only breakage would be construction issues. Link to comment Share on other sites More sharing options...
oldlady Posted November 25, 2019 Report Share Posted November 25, 2019 soft brick can be cut so easily that you might just cut some slices and pick up the tile and the slice of brick together. Link to comment Share on other sites More sharing options...
douglas Posted November 25, 2019 Report Share Posted November 25, 2019 My bet is that you are putting the tiles on hot brick, and the thermal shock is what is cracking them during firing. Try using cold brick to rest the tile on and see if that helps. Link to comment Share on other sites More sharing options...
Recommended Posts
Archived
This topic is now archived and is closed to further replies.