Mbdale Posted November 24, 2016 Report Share Posted November 24, 2016 Has anyone seen this before? Clay is speckled brownstone and was fired to cone 6. I have an electric kiln with a kiln sitter. Every peice in my last load did this, except the peices made with little loafers. Link to comment Share on other sites More sharing options...
Mark C. Posted November 25, 2016 Report Share Posted November 25, 2016 Classic bloating Whats the temp this clay is rated to ? Link to comment Share on other sites More sharing options...
glazenerd Posted November 25, 2016 Report Share Posted November 25, 2016 Has anyone besides me noticed the marked increase in stoneware/clay issues-complaints? (last 2-3 months) Nerd Link to comment Share on other sites More sharing options...
neilestrick Posted November 25, 2016 Report Share Posted November 25, 2016 Has anyone besides me noticed the marked increase in stoneware/clay issues-complaints? (last 2-3 months) Nerd Yes, but is it due to more problems or just increased traffic on the forum? Link to comment Share on other sites More sharing options...
glazenerd Posted November 25, 2016 Report Share Posted November 25, 2016 If I had to put numbers on it: 60/40. 60 from new traffic, 40 existing members. However, of the 60%, most seem to be firing to what is recommended by maker. Nerd Link to comment Share on other sites More sharing options...
bciskepottery Posted November 25, 2016 Report Share Posted November 25, 2016 Classic bloating Whats the temp this clay is rated to ? Speckled Brownstone is a cone 4 to 6 clay body; at cone 6, less than 1% absorption. Wondering if the kiln over-fired? Or a long hold at peak temperature? Link to comment Share on other sites More sharing options...
neilestrick Posted November 25, 2016 Report Share Posted November 25, 2016 Under 1% is mighty tight for a stoneware body. You'll need to put some cones on the shelf in your next firing to confirm the heat work. Could be your sitter is out of calibration. Link to comment Share on other sites More sharing options...
bciskepottery Posted November 25, 2016 Report Share Posted November 25, 2016 A number of Highwater's stonewares for cone 6 are 1% or less for absorption . . . Red Rock, Red Stone Speckled Brownstone, Riverside Grit; they also have shrinkage rates of 10%. Link to comment Share on other sites More sharing options...
Marcia Selsor Posted November 25, 2016 Report Share Posted November 25, 2016 definitely bloating which usually means overtired or gases did not fire out in bisque and were trapped in a fast firing during the ^6 firing in simple terms.How fast was the glaze firing? And how high did you bisque and how fast? Marcia Link to comment Share on other sites More sharing options...
Mark C. Posted November 25, 2016 Report Share Posted November 25, 2016 What temp did you bisque it to? Did you hold it at cone 6 for some time is what Neil is asking Nerd the summer traffic is slow and when school starts up the site gets all kinds of inquires on issues with clay and glazes then gets quiet again around the Holidays X-mas and new years .Then picks up until the summer break and its sleepy time again. I have seen mostly clay issues in cone 6 bodies for years here and stoneware is the leader for sure. Not my area of experience but is appears many clay makers make clay that has to broad a temp range-hence the leaking pots and glaze issues. Together with commercial glaze application issues and its a stew of discontent at times. Link to comment Share on other sites More sharing options...
glazenerd Posted November 25, 2016 Report Share Posted November 25, 2016 clay makers make clay that has to broad a temp range Stoneware is much more critical and unforgiving in regards to KnaO and silica levels compared to porcelain. Porcelain will melt in direct proportion to the amount of silica and KnaO: stoneware will not. Stoneware depends upon mullite to vitrify. As mullite forms: it will eject excess silica not required to form a grain of mullite. The excess silica then causes dunting, so they increase KnaO levels in an attempt to compensate: then that excess can cause bloating. Makers also rely too much on iron as a flux: which holds true for cone 10 reduction firings: but only partly true for cone 10 oxidation, and even less true for cone 6. As Bruce pointed out a broad cone range: I think in this case they increased the flux for use in the lower cone; which in turn created a bloating problem at higher cones. For those using broad range cone rated clay: use very long extended holds at the low end of the cone rating, and very short holds at the high end. Nerd Link to comment Share on other sites More sharing options...
Mark C. Posted November 25, 2016 Report Share Posted November 25, 2016 It appears the broader the range the broader the problems. Link to comment Share on other sites More sharing options...
Babs Posted November 25, 2016 Report Share Posted November 25, 2016 New clay test it at what you want to do with it prior to making kilnlosd Link to comment Share on other sites More sharing options...
bciskepottery Posted November 25, 2016 Report Share Posted November 25, 2016 Given the pinholing on the glaze work and the blisters on the bottom, I am thinking the problem lies in bisque firing -- bisque at cone 04 and add a hold at peak of about 10 minutes to allow manganese and other gases enough time to burn off. Also, use a slow glaze schedule with a top hold. Speckled Brownstone has manganese in it to give the speckled appearance. Link to comment Share on other sites More sharing options...
glazenerd Posted November 25, 2016 Report Share Posted November 25, 2016 Mark: absolutely agree; the clay industry needs to abandon the practice of broad cone range certifications. Bruce: I partly agree, bisq temps play a role in carbon burn off: and I agree with the longer hold at peak bisq temps. These particular blisters remind of me high calcium issues, have seen them before in experimental clay bodies. Manganese however I doubt played much of a role. Manganese goes through a phase change at 2275F; from solid to liquid. I bisq porcelain to 1840F and stoneware to 1880F; which still leaves them porous to accept glaze. I think your original post of being over-fired is the most likely cause. Nerd Link to comment Share on other sites More sharing options...
Mbdale Posted November 25, 2016 Author Report Share Posted November 25, 2016 Bisqued to cone 04 used this schedule with a downdraft Preheat 8 hours bottom on low lid propped Top peep hole is open Drop lid all to low 4 hours, all medium, 4 hours, all high 4 hours The sitter dropped at 4 hours Glaze cone 6 Preheat 2 hours btm on low Top peep hole is open Low 3 hours Medium 4 hours High 3 hours It took about 15 extra minutes for the sitter to drop Link to comment Share on other sites More sharing options...
Min Posted November 25, 2016 Report Share Posted November 25, 2016 I know colours can be off when looking at pictures online but when I compare your raw clay colour to this image of that clay your clay looks quite overfired, I have found that when medium iron bodies get overfired they get that old dead meat colour your pot bottom has. Witness cones are super helpful in checking and monitoring the kilns actual firing. Link to comment Share on other sites More sharing options...
Mark C. Posted November 25, 2016 Report Share Posted November 25, 2016 How do you know it was fired to cone 6-did you have a cone in kiln? or just looking at kiln computer readout?? Was the cone sitter the only cone? Link to comment Share on other sites More sharing options...
bciskepottery Posted November 25, 2016 Report Share Posted November 25, 2016 +1 to Min for "dead meat look" . . . Link to comment Share on other sites More sharing options...
neilestrick Posted November 25, 2016 Report Share Posted November 25, 2016 A number of Highwater's stonewares for cone 6 are 1% or less for absorption . . . Red Rock, Red Stone Speckled Brownstone, Riverside Grit; they also have shrinkage rates of 10%. Have you ever verified those numbers? With absorption rates that low I would expect to see higher shrinkage numbers, except maybe the Riverside Grit which is high in grog. All the stoneware bodies I have used are closer to 2% absorption and 12% shrinkage. Link to comment Share on other sites More sharing options...
bciskepottery Posted November 25, 2016 Report Share Posted November 25, 2016 A number of Highwater's stonewares for cone 6 are 1% or less for absorption . . . Red Rock, Red Stone Speckled Brownstone, Riverside Grit; they also have shrinkage rates of 10%. Have you ever verified those numbers? With absorption rates that low I would expect to see higher shrinkage numbers, except maybe the Riverside Grit which is high in grog. All the stoneware bodies I have used are closer to 2% absorption and 12% shrinkage. I've not verified them through testing. I have used Red Rock for a number of years, mostly because the absorption is lower than Standard 112. Have had no issues with seepage, etc. The shrinkage rate seems what it is advertised . . . from experience using RR and Little Loafers and a shrink rule for measuring slabs. Link to comment Share on other sites More sharing options...
Marcia Selsor Posted November 26, 2016 Report Share Posted November 26, 2016 Have you talked with High-water?Once in a while clay producers can get a bad shipment of raw materials that no one has noticed it has changed. If this is new to the clay, they may have a problem. Call them and ask if others are experiencing this. Many years ago another producer had a bad batch going into their earthenware body. Calcium pitting ruined a lot of pots. Marcia Link to comment Share on other sites More sharing options...
curt Posted November 26, 2016 Report Share Posted November 26, 2016 clay makers make clay that has to broad a temp range Stoneware is much more critical and unforgiving in regards to KnaO and silica levels compared to porcelain. Porcelain will melt in direct proportion to the amount of silica and KnaO: stoneware will not. Stoneware depends upon mullite to vitrify. As mullite forms: it will eject excess silica not required to form a grain of mullite. The excess silica then causes dunting, so they increase KnaO levels in an attempt to compensate: then that excess can cause bloating. Makers also rely too much on iron as a flux: which holds true for cone 10 reduction firings: but only partly true for cone 10 oxidation, and even less true for cone 6.As Bruce pointed out a broad cone range: I think in this case they increased the flux for use in the lower cone; which in turn created a bloating problem at higher cones. For those using broad range cone rated clay: use very long extended holds at the low end of the cone rating, and very short holds at the high end. Nerd As we clarified in the "Stoneware Limit" thread recently, both porcelain and stoneware have both mullite and glassy phase when vitrifying. They both melt the same way. Neither depends more or less on KNaO or any other flux than the other. Seems like some of the old thinking creeping back in... Bloating could result from overfiring with either body. However I would say that it is the higher purity of the ingredients in a typical porcelain body that makes bloating less likely with porcelain, where warping is a much more common evidence of overfiring. Mullite formation doesn't "eject" silica. The excess fine free silica which can result in excessive cristobalite is where the dunting problems come from. This excess free silica is either already present in the body or it is not from the moment it is blended. Given that heatwork (the combination of both temperature AND time) is what produces vitrification, and that many combinations of time and temperature can produce vitrification in a given clay body, I am not sure that extended holds at the low end of the advertised firing range is really a remedy for excessive flux levels. If a clay has too much flux for a given cone, then it just has too much flux - no matter what combination of time and temp one uses to accomplish the heatwork. Bloating is usually caused by insufficient burnout of organic matter present in the clay during the bisque firing. This burnout takes place at around Orton Cone 015, not cone 05 or 06. (ie, around 800C, not 1000 or 1050C). Try a soak for an hour at around 800C during the bisque on the way up and if organic or carbonaceous matter is the problem that should clear it out. Also make sure the kiln is well ventilated, all bungs out, maybe lid cracked, etc. during bisqueing to make sure there is plenty of oxygen for the burnout to be accomplished. And finally, yes broad cone range is code for "our customers don't know any better so let's telll them that our clay is 'one size fits all'". Without everyone investing in their own Nerd-like research programs, I am not sure if there is any easy way to make the average potter more aware that one size does not and never could fit all... Link to comment Share on other sites More sharing options...
glazenerd Posted November 26, 2016 Report Share Posted November 26, 2016 Curt: Ceramic Science for the Potter Lawrence & West 1982 (2nd Edition) makes the statement about spinel ejecting excess silica. Mullite is a glassy matrix, just a different kind. Try not to get overly specific in my posts; people probably go to sleep reading my posts.. boring! Nerd Link to comment Share on other sites More sharing options...
curt Posted November 26, 2016 Report Share Posted November 26, 2016 I don't have Lawrence and West, will have to procure a copy. Anyway, I guess one could say that silica is a product of the spinel phase if that is what you meant. However, not sure mullite could be called a glassy matrix since mullite is crystalline and glass by definition is amorphous. Maybe you mean mullite crystals are present within the glassy matrix? Does any of this matter? Only if it leads to the wrong conclusions.... Link to comment Share on other sites More sharing options...
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