kristinanoel Posted June 4, 2020 Report Share Posted June 4, 2020 Hi everyone - I've spent the last few hours combing through the forum to learn more about the factors to consider in setting up a firing schedule, I've come up with a plan and would love to hear feedback, and/or anywhere I've gotten the wrong end of the stick. I'm bisque firing to 04, glaze firing to 5 in an electric kiln with a bartlett controller. Any advice is welcome! 04 Firing Schedule Segment 1 Goal bring ware to even, pre-qartz inversion temperature target temp 1000° temp change ~935° Rate 325°/hr Hold 10 minute even core / surface temp Approx. time 3 hours Segment 2 Goal slow ramp for quartz inversion target temp 1100° temp change 100° Rate 100°/hr Hold 0 Approx. time 1 hr Segment 3 Goal allow sufficient time for organic materials to burn out of clay target temp 1925° temp change 815° Rate 125°/h Hold 15 minute soak to even surface/ core temperature Approx. time 7 hrs Total time ~ 11 hrs Questions: Is the first segment too fast? Assuming ware has been candled and there is no atmospheric water, what am I risking? Can I speed up segment 3? Seems inordinately long, but I am trying to 1) give enough time for organic materials to burn out, especially as I use dark clays, and 2) accommodate the target rate range for the last ~150° so that I can get accurate readings of the cones. Anything glaring I didn't know to ask? Cone 5 Firing Schedule Segment 1 Goal dry any atmospheric water from glazing target temp 300 temp change 235° Rate 150°/hr Hold 0 Approx. time 1.5 hours Segment 2 Goal bring ware to even, high bisque temp target temp 1925° temp change 1625° Rate 325°/hr Hold 20 minutes to even core / surface temp Approx. time 5 hours Segment 3 Goal slow ramp for sodium/potassium offgassing target temp 2150° - (mid cone 5 range of 2118° - 2205°) temp change 225° Rate 125°/hr Hold 10 minutes to even core / surface temp Approx. time 2 hrs Total Time 9.5 hours Questions: Do I need a Quartz Inversion segment in glaze firing? In some of the sample schedules I've seen them, but have also read that this is more of an issue in the first firing? Any other reason to break up or slow down that middle segment? It seemed like the goal was to pull the ware up to bisque temp asap, and then slow down for clay and glaze maturity, but is there something I've missed? Anything glaring I didn't know to ask? All comments welcome! Kristina Link to comment Share on other sites More sharing options...
Bill Kielb Posted June 4, 2020 Report Share Posted June 4, 2020 5 hours ago, kristinanoel said: Hi everyone - I've spent the last few hours combing through the forum to learn more about the factors to consider in setting up a firing schedule, I've come up with a plan and would love to hear feedback, and/or anywhere I've gotten the wrong end of the stick. I'm bisque firing to 04, glaze firing to 5 in an electric kiln with a bartlett controller. Any advice is welcome! Here are some tried and true automatic cone fire programs to cone 04 and cone 6. Quartz inversion just happens, clay is pretty sturdy and can take it. These schedules have been run in thousands and thousands of firings over the last twenty years. These are the automatic cone fire schedules that run in the Bartlett controller, which has made controls for nearly all kilns in the US. Adjust your ending temp as necessary. When writing your own glaze fire, subtract 250f from the final temp and set this segment at about 110 degrees per hour to get your cone to bend near the 108 degree per hour Orton cone chart. In general, quartz inversion just happens and when you get to someone talking cristobalite inversion, in my opinion it’s very rare to have it in most clays. One thing that is always important is to dry everything out thoroughly which means some time at 200 or below., slowly. Link to comment Share on other sites More sharing options...
neilestrick Posted June 4, 2020 Report Share Posted June 4, 2020 The pre-programmed cone fire profiles work very well. No reason for using custom schedules, especially in bisque, unless you're looking for some specific result. Link to comment Share on other sites More sharing options...
kristinanoel Posted June 4, 2020 Author Report Share Posted June 4, 2020 Thanks for replying! More questions, if you're up for them? When I compare the two schedules, there are a couple of differences in the bisque thatI'd like to understand. The cone 5/6 schedules aren't materially different (slightly lower target for segment 1, faster for segment 2, and higher temp to end). For the bisque: In the initial temperature rise to 1000°, the auto schedule starts at a slow rate of 80°/hr to 250°, then includes a med ramp of 200°/hr to 1000. What's happening with the clay here? is there some reason this stage needs 6 hours rather than 3? Post quartz inversion slowdown, the rate of 180°/hr is higher than I would have expected to burn out sulfides and organic materials. Is that faster speed sufficient for darker clay bodies? What's going on in that final slower 80°/hr segment? Just for temperature regulation, or something else? Segment 1 Segment 1 What's going on here? slow warm to elements or for drying clay? Goal bring ware to even, pre-qartz inversion temperature Goal target temp 1000° target temp 250 temp change ~935° temp change 185 Rate 325°/hr Rate 80 Hold 10 minute even core / surface temp Hold Approx. time 3 hours Approx. time 2.25 Segment 2 slower rate - why? is something going on with the clay? Goal bring ware to pre quartz inversion target temp 1000 temp change 750 Rate 200 Hold Approx. time 3.75 Segment 2 Segment 3 no difference Goal slow ramp for Quartz Inversion Goal slow ramp for Quartz Inversion target temp 1100° target temp 1100° temp change 100° temp change 100° Rate 100°/hr Rate 100°/hr Hold 0 Hold 0 Approx. time 1 hr Approx. time 1 hr Segment 3 Segment 4 faster ramp rate - why? Goal allow sufficient time for organic materials to burn out of clay Goal target temp 1925° target temp 1695° temp change 815° temp change 815° Rate 125°/h Rate 180°/h Hold 15 minute soak to even surface/ core temperature Hold Approx. time 7 hrs Approx. time 3.3 hrs Total time ~ 11 hrs Segment 5 why so slow? just so that no hold is needed, or some other reason? Goal bring ware to temp evenly between surface/core target temp 1945° temp change 250° Rate 80°/h Hold Approx. time 3.1 hrs Total time ~ 13 hrs Thanks! Link to comment Share on other sites More sharing options...
Min Posted June 4, 2020 Report Share Posted June 4, 2020 @kristinanoel, there is a very good article and schedule for bisque firing black and dark brown high manganese bodies here. I do think the firing needs to slow down during the periods that the carbons are burning off. That would be between 300F- 600F for the organic carbons and between aprox 1300F and 1650F for the inorganic carbons. For bisque firing I don't think slowing down in the 1000F - 1100F range is necessary. Link to comment Share on other sites More sharing options...
kristinanoel Posted June 4, 2020 Author Report Share Posted June 4, 2020 Thanks, Min - that was a great article! Because I do work with iron rich clays, I'm adding additional time for that organic carbon burn off between 300 - 600, and keeping the slower ramp for the inorganic. On the 1000F - 1100F quartz inversion slowdown - such a lot of conflicting information here. Most of the what I've read has included this slowdown phase in the bisque but not in the glaze, as in the example that Bill posted. My understanding was that the focus for the glaze firing is to achieve maturity for both clay and glaze in the ~1950F - 2190F range, hence the fast ramp to high bisque temperature. From what I have learned, it seems that the majority of problems occur on the way down from 1100F to 1000F, as there is less available room for movement. Has that been your experience? I've not experimented with slowing the cooldown, but may incorporate that into future firings. Link to comment Share on other sites More sharing options...
Bill Kielb Posted June 4, 2020 Report Share Posted June 4, 2020 1 hour ago, kristinanoel said: Thanks, Min - that was a great article! Because I do work with iron rich clays, I'm adding additional time for that organic carbon burn off between 300 - 600, and keeping the slower ramp for the inorganic. On the 1000F - 1100F quartz inversion slowdown - such a lot of conflicting information here. Most of the what I've read has included this slowdown phase in the bisque but not in the glaze, as in the example that Bill posted. My understanding was that the focus for the glaze firing is to achieve maturity for both clay and glaze in the ~1950F - 2190F range, hence the fast ramp to high bisque temperature. From what I have learned, it seems that the majority of problems occur on the way down from 1100F to 1000F, as there is less available room for movement. Has that been your experience? I've not experimented with slowing the cooldown, but may incorporate that into future firings. I’ll just give what might be a controversial opinion here. Slow bisque wherever you put the speed is a good idea as initially we want to dry out the physical water slowly, then carbon burnout, but we forgot to include removing chemically bound water and of course there is quartz inversion. So lots of change happening and some contaminants take time and temperature to burnout, not just temperature alone. As far as quartz inversion it does exist and happens during bisque and glaze firings. Up and down. It’s interesting that production tile goes from room temp to cone ten and back again in hours. Too fast? Clay is pretty tough stuff so worrying about quartz inversion is great, but likely not as deadly as many believe. generally bisque schedules don’t exceed 300 degrees per hour. If your clay is exceptionally dirty then I would expect to fire slower and longer. The cone fire schedules used by Bartlett have proven pretty reliable for many years and many claybodies. Unless I had a particular body requirement it seems their slow bisque has been very reliable. Quartz inversion? It exists but then again clay has proven tough.. Link to comment Share on other sites More sharing options...
kristinanoel Posted June 4, 2020 Author Report Share Posted June 4, 2020 Thanks Bill! I'll always listen to an opinion. That's exactly why I posted. And Just to be clear - I'm not doubting the reliability or efficacy of the Bartlett program, I ask questions so that I can develop a better understanding of the reasons behind the choices. For example... that standard limit of 300F per hour - I reckon that's to smooth the transition for atmospheric or chemically bound water? Link to comment Share on other sites More sharing options...
Min Posted June 4, 2020 Report Share Posted June 4, 2020 I think there are at least a couple things being talked about here. When we are talking about the (sometimes) problems with fast heating or cooling through the 1000F - 1100F temperature range, quartz inversion, we are bringing up a discussion on cristobalite. If we are talking about cristobalite then we need to look at the 3 main sources of silica found in clay bodies. There is the ground silica/quartz, the free silica which is inherent in any clay plus silica that forms when kaolinite breaks down into mullet plus free silica. Silica glass forms as the fluxes melt and incorporate silica, this form of silica has a very low thermal expansion. However crystalline silica has the opposite characteristic; it has a very high rate of thermal expansion. Lastly cristobalite inversion has an even higher rate of expansion/contraction which happens in the 400F range. Mesh size of the quartz is super important plus the type of silica present. It has been shown that cristobalite can form between 2012 - 2102F if there are fine particles of silica present plus some sodium carbonate then cristobalite can form within 2 hours. This could mean an extremely slow firing or prolonged soaks in this temperature range has the possibility of cristobalite formation in bodies favouring it. I remember being taught that cristobalite wasn’t an issue with cone 6 clay but that doesn’t seem to be the case. Also, if you refire these type bodies it will increase the amount of cristobalite present. Once present it can’t be removed. Well vitrified, well blended bodies can avoid cristobalite development as can the speed of firing. You’re not going to find cristobalite in a porcelain but it could be present in high iron non vitreous stoneware. If there is cristobalite present then slowing down the rate of firing/cooling of the glaze firing during the 1000 - 1100F range could be prudent. More so with the cooling as the clay has reached maturity. On bisque firings the quartz inversion happens at the same time as the shrinkage of the body occurs therefore I think there is a counteracting mechanism at play. I don't think it's much of an issue for most pots, larger pieces and sculpture would probably be safer to slow down the firing during this range for. However for re-fires, going slow through this range would be prudent. If there is cristobalite present in the body the 400 - 450F range should be slowed down also, both with heating and cooling. Only way to measure cristobalite is with a diameter. More mechanisms at plays to avoid thermal shock than just cristobalite though. Link to comment Share on other sites More sharing options...
Bill Kielb Posted June 4, 2020 Report Share Posted June 4, 2020 1 hour ago, kristinanoel said: Thanks Bill! I'll always listen to an opinion. That's exactly why I posted. And Just to be clear - I'm not doubting the reliability or efficacy of the Bartlett program, I ask questions so that I can develop a better understanding of the reasons behind the choices. For example... that standard limit of 300F per hour - I reckon that's to smooth the transition for atmospheric or chemically bound water? I take it as: you can go as fast as you like as long as your claybody doesn’t suffer any issues from incomplete burnout. Most bisque schedules take from 9- 12 hours so the math says you generally won’t see faster than About 300 per hour. Is that the number that keeps things all good? I wouldn’t speculate other than there are so many things going on early in a bisque firings that some relative slow speed is likely cautious. When I look at schedules and do the math as well, approximately 300 degrees per hour is probably the fastest I see. During the development of a gas firing digital aid which told you what speed you were going I now found myself needing this answer! I spent over two years asking folks how fast should we gas fire? In the beginning I got answers from 5- 150. degrees per hour. It was like those questions around thanksgiving where young kids are asked how long to cook the turkey. 30 hours at 5 degrees! I saw lots of published material presented citing quartz inversion, cristobalite squeeze, chemically combined water ......... on and on. Finally I found some interesting info about production firing going 1000 degrees per hour or more and what I would call a very sensible ceramics / material science professor that just flat out said ...... yeah Quartz inversion it happens all the time. Most of the time the piece is going through it unevenly as the inside of the pot is degrees different than the outside or from top to bottom often there is a significant temperature gradient.............And then he said something that made my head spin ........... cristobalite! ........ You likely just ain’t gonna have it with any significance with typical pottery. I said hah, what about refires? He laughed and said how often did I fire something to above cone for significant time and then refire it again? I said ok I get the point, I will stop worrying about cristobalite For the most part I am not gonna have it. Now back to our two year long experiment with a gas fired kiln that fired early, 300 degrees hotter on top than the bottom. Later it flipped after reduction (About 1700 degrees) and fired 50-100 degrees hotter on the bottom than the top. So each and every firing we started slowwwwwww and by the end everyone was clamoring to go 500 degrees per hour because we never ever cracked a typical pot over many, many firings. So what speed is good? Frustrated, I stated my dilemma with wanting the official number, what was it? The only comment I got back is quartz inversion happens, generally clay can take it ........ room temperature to cone ten to room temperature in hours. I said thanks, I got it now. The point I think he was trying to make is there are likely limits, claybody dependent, thickness dependent, the possibility of crystobalite .... but in the end he told me generally it's not the kilns fault, make better pots and use reasonable caution in your schedule. Or in my case don’t be so fixated by a single number, the processes are proportional or analog if you will. Still If we could have gotten that old gas kiln up to 600 degrees per hour During midfire we would have done it though. Not sure that helps, but a true story And a fun two years without ever finding the magic number. Link to comment Share on other sites More sharing options...
kristinanoel Posted June 4, 2020 Author Report Share Posted June 4, 2020 "you can go as fast as you like as long as your claybody doesn’t suffer any issues from incomplete burnout. " ha! Makes sense - also makes sense that the ranges to watch are between 300 - 600F for organics, ~1200F - 1750F for sulfides and inorganic carbons. I find it more helpful to think about what each segment is trying to do, less helpful to identify the "magic number", as you so cleverly put it! Thank you for your story. Sounds like you made some really good memories and a lot of good friends. Link to comment Share on other sites More sharing options...
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