Marilyn T Posted February 14 Report Share Posted February 14 Background: I'm trying to stack the odds in my favour for determining the "sweet spot" on a drop and hold temperature between peak (2185) to 1750 (when glaze is no longer fluid.) I am using the MC6 glaze firing profile with two 10 min holds (2080 and 1900) which works for two of my glazes (Variegated Blue and Rutile Green), but the third glaze (Wollastonite Clear) is alluding me (only a few pinholes -- sometimes on outside at hold at 2080 and last firing with hold at 2040 for 30 min. the outside was defect free but the inside had two pinholes. Note: I've had the most success with the MC6 firing schedule so want to use that as a base schedule). I'm attaching a breakdown of the viscosity and surface tension amounts of the WC glaze. Does this help to predict the "sweet spot" hold temperature? ViscositySurfacetension.pdf Quote Link to comment Share on other sites More sharing options...
Bill Kielb Posted February 15 Report Share Posted February 15 (edited) This is the way I learned this particular pinhole theory but to my knowledge and what I have been taught, like fired COE, the fired characteristics cannot be obtained from the glaze formulation and measuring at temperature for just one composition has its own complications. + one from me realizing kilns generally don’t have powered cooling cycles so they cool at their rate (thermal mass being significant here) and you simply establish holds along the way. Multiple holds can give you step control though so closer to a defined rate of cooling. To my knowledge alleviating the surface tension issue involves trial and error though. Some of the randomness to multiple trials could be the different natural rates that things are actually cooling. So if your kiln cools very quickly at top end, then your holds could be more material and reliable. Hopefully I am wrong and someone here has the magic, but from my experience testing is necessary. Pinholes can have other causes though so you may want to explore those cures as well. I will add that temperature differences from shelf to ware or outside of a ware to inside of an open shape always seemed highly unlikely just because ceramic conducts heat pretty quickly, generally much faster than typical heating and cooling rates it is subjected to. Still, if you find a solution based in that theory then it works for you. I believe you need to test to find the best answer for a glaze combination. One solution I really liked was to fire short of peak temp using a hold to still make cone heatwork and doing a drop and hold from there. In my experience glazes can pinhole more so at higher firing temps. So firing one cone lower with a fifteen to 20 minute hold drives the heatwork to the desired cone but without the peak temperature. Something maybe worth trying with your established drop and hold from there. Finally, if this reads crazy complicated, it’s just me not being able to adequately simplify, someone likely will have a simple fix. Edited February 15 by Bill Kielb Piedmont Pottery 1 Quote Link to comment Share on other sites More sharing options...
Min Posted February 15 Report Share Posted February 15 @Marilyn T, could you post the recipe, glaze specific gravity and which clay you are using? Also, does firing to 2185/2190F bring cone 6 down in your kiln? No soak at peak temp? Marilyn T 1 Quote Link to comment Share on other sites More sharing options...
Marilyn T Posted February 15 Author Report Share Posted February 15 Thank you Bill and Min for your replies. I appreciate any ideas you put forth as I have a 100+ dinner set ready to glaze in a white. I bisque at ^4. 1. recipe attached. This was the best of 20 recipes I mixed up and fired. The MC6 firing schedule also gave the least pinholes. Other schedules were C5 and 6 of Digitalfire C5DHSC; Naomi Clement's schedule; and Digitalfire PLC6DS. The next best performing recipe was Studio White. 2. Glaze specific gravity 1.40 3. Clay: Laguna #613 clay has a 5.91 COE. and fired cone is: (5 scrunched on shelf; 6 is 4:00 position and 7 is not bent). My aborbency testing on this clay is 0.2 at cone 5; 0 at cone 6 and 6.6 at cone 7. 4. When testing on 1 lb. bowls, my firing schedule is changing every two days, but the last firing I did which had no pinholes on the outside and two pinholes on the inside (photo attached) was: Segment Rate Temperature Hold Notes 1 100 220 30 2 350 2000 0 3 108 2190 5 4 9999 2040 30 (after feldspar off-gassing finishes) 5 100 1400 off (1750 glaze no longer fluid). Previous firings: Lower top temperature had more pinholes. A previous soak at 2080 had 5 pinholes on outside opposite element and none inside. The latest firing (after the one above) with a soak at 2030 with another one of 10 minutes at 1900 had 3 pinholes on the outside next to the element and none inside. I only fired that one down to 1700. The other glazes on this clay, that I fired successfully have a drop to 2090 with only a 5 minute hold. They are Variegated Blue and Rutile Green (both fluid glazes). Thanks for any help you can give me on this. Hulk 1 Quote Link to comment Share on other sites More sharing options...
Min Posted February 15 Report Share Posted February 15 (edited) Really good information Marilyn, thank you for such a comprehensive post. I'm not seeing anything really glaringly off with the recipe or the glaze firing. Glaze recipe looks like it would make a very durable glaze. I haven't used that claybody but I'm wondering if how it's bisque fired might be part of the problem. Is your kiln vented and do you really cram the bisque firing full? How fast do you bisque fire to 04, using a preprogrammed schedule? Do you have a closeup of a couple of the pinholes? Edited February 15 by Min grammar Hulk 1 Quote Link to comment Share on other sites More sharing options...
Hulk Posted February 15 Report Share Posted February 15 The circled defects (posted image by OP, above), is the bare clay visible? Are the other(smaller) spots also defects? ...looks like tiny pits in the glaze. The circled spots look like what I've called "large particle defect" - where something in the clay was off gassing, something that wasn't burned out in the bisque. I'd added a hold at 1500°F to the bisque when experimenting with black and dark red clays*; it seemed to help, a lot. One full hour, or half hour on the way up and again on the way down. Since, I've left the bisque hold, against the occasional "large particle defect" which seems to happen more or less often, depending on the clay... Running a powered kiln vent - supplying oxygen - should be helping with the burnout*. Are there more defects where the clay is thicker? More defects in thicker/heavier pieces? The blue and green glazes aren't showing any defects? *credit to Forum member GlazeNerd PeterH and Callie Beller Diesel 2 Quote Link to comment Share on other sites More sharing options...
PeterH Posted February 15 Report Share Posted February 15 1 hour ago, Hulk said: The circled spots look like what I've called "large particle defect" I've been having vaguely similar thoughts about larger wollastonite particles. I'm not suggesting that wollastonite particles can out-gas, but could their presence influence the seeding and/or movement of bubbles? Quote Link to comment Share on other sites More sharing options...
Marilyn T Posted February 15 Author Report Share Posted February 15 Thank you Min, Hulk and Peter for replying with your insights. Hopefully, with your help, I may be closer to a solution. To reply to Min first: Re: bisque schedule - I use Naomi Clement's 17 hour bisque schedule to 04 and only have a few pieces in the middle of the kiln as I am just doing testing at the present time. My kiln is well vented (by Envirovent) - flame test okay. Re: glaze schedule: Close up picture of pinhole attached. Hole is an "innie" and there is brown clay spit out on the outside of the hole. My guess was that it was feldspar which is why I started my glaze holds below 2050. Firing to 2050 on Clement's glaze schedule had a few pinholes and the glaze was not fully melted, so I had to go to 2180-2190 (cone 6) to get full melt. (Katz boron chart). On her schedule with a 2190 peak had only two pinholes inside the bowl, and the glaze was fully melted. Reply to Hulk: (my reply in purple) The circled defects (posted image by OP, above), is the bare clay visible? Yes Are the other(smaller) spots also defects? ...looks like tiny pits in the glaze. They are smooth but yes visible (possibly healed) The circled spots look like what I've called "large particle defect" - where something in the clay was off gassing, something that wasn't burned out in the bisque. Sounds good to me. I'd added a hold at 1500°F to the bisque when experimenting with black and dark red clays*; it seemed to help, a lot. One full hour, or half hour on the way up and again on the way down. I will try this on the next firing. This is Naomi's 17+ hr. bisque schedule with your suggestion in it. Look okay? 1890/180/ 1 hr. hold 50/200/10 min hold 150/300/0 hold 100/1525/1 hour hold. This was already part of her schedule. 75/1750/0 hold 108/1930/10 min (original was 100/1920/10) but I changed it to 108 so the cone reading was more accurate. 9999 to 1500/ 30 min hold. ??rate Since, I've left the bisque hold, against the occasional "large particle defect" which seems to happen more or less often, depending on the clay... Running a powered kiln vent - supplying oxygen - should be helping with the burnout*. Are there more defects where the clay is thicker? not on the bowl I was using. Haven't done larger pieces yet. More defects in thicker/heavier pieces? There was no defects on the test tiles, but these appeared on the 1 lb. bowls (haven't gone bigger yet). The blue and green glazes aren't showing any defects? No. The fluidity of the glaze seems to fill the holes and because there are colorant they aren't visible. Reply to Peter: I could try Whiting instead of Wollastonite to test this theory. It would also make the glaze a bit more fluid but the LOI would go up from 3.1 to 6. 3. I'll wait to hear what Hulk and/or Min says on this before trying it. Hamer & Hamer has the melting point of wollastonite at 2813F and I don't know its melting behaviour in a mid range glaze, but I'd like to know. Thanks all for your help with this. The dish set was supposed to be a Christmas present so I'm long overdue on gifting it. They specified white on brown clay, so I have to stay the course. Cheers. Hulk 1 Quote Link to comment Share on other sites More sharing options...
Min Posted February 15 Report Share Posted February 15 Okay, correct me if I got any of this wrong: Lower top temp + 5 min soak = more pinholes. You are dropping to 2040F then soaking for 20 = best results so far Bisque schedule looks great. Pinholes look they could very well be from clay contaminates that haven't burned out in bisque. I would suggest dropping your top temperature to something in the range of 2165F, or enter ^5 for your peak, then add a 18 minute soak, this should get you close to cone 6. Then I would drop to 2085 and do another soak, this one for 12 minutes. If the glaze does not need a slow cool to develop the visual texture, ie it's a gloss glaze with no streaking / variation then I would shut if off then. If it, or other glazes in same firing, need a slow cool below 2085 then follow your regular downfiring schedule. This suggestion is just somewhere to start, do your usual cones, see what this schedule gets them to and just try a piece or two to see what happens. I'ld also suggest adding a bit of thickish dry glaze paste (the stuff from around the edge of the bucket) to the pinholes and stick the pot back in the kiln for another firing, see if the pinholes heal over or get worse. (put this pot in the coolest part of the kiln) I know how frustrating it can be to fire a kiln nearly empty (apart from the typical number of shelves also in the kiln) but it saves tossing a load of pots with pinholes. Did the other glazes, including Studio White result in pinholes also? Quote Link to comment Share on other sites More sharing options...
PeterH Posted February 15 Report Share Posted February 15 (edited) 1 hour ago, Marilyn T said: Reply to Peter: I could try Whiting instead of Wollastonite to test this theory. It would also make the glaze a bit more fluid but the LOI would go up from 3.1 to 6. 3. I'll wait to hear what Hulk and/or Min says on this before trying it. Hamer & Hamer has the melting point of wollastonite at 2813F and I don't know its melting behaviour in a mid range glaze, but I'd like to know. It was a very tentative suggestions and I would certainly wait to hear what the experts say. If the idea seems worth testing I would start (and probably end) with ensuring that the wollastonite is well sieved, preferably with one of the additives mentioned. Personally for serious test I would mix with the additive and slake overnight before and after sieving to allow time for any surface "interactions" to occur. Maybe try with a cup of glaze taken from your well-stirred bucket. But with your low density of defects you'll need to dip a lot of test-tiles to cover the area required! PS Glaze Pinholes, Pitting https://digitalfire.com/trouble/glaze+pinholes%2C+pittingI If pinholes are isolated and few in number it may be possible that a contaminant is getting into the glaze. Pour a sample through a fine screen to check. Do not underestimate the value of ball milling to improve fired glaze surface qualities, many a problem with pinholing and blistering has been solved this way. Many companies ball mill up to 12 hours for best results. Edited February 15 by PeterH Quote Link to comment Share on other sites More sharing options...
Marilyn T Posted February 15 Author Report Share Posted February 15 Thank you both for your suggestions. Min: 1. You asked if this was correct: Lower top temp + 5 min soak = more pinholes. You are dropping to 2040F then soaking for 20 = best results so far YES. Out of the box: I did try a 25 hour adaptation of the NC firing schedule by building in a very slow cool from 1800 to 1400 but the ware warped and the glaze was very dry with no pinholes. All the cones 5 6 7 were melted down. Same with the MC6 schedule taken to a high temperature 2232. I had no pinholes but there were numerous small bumps (blisters) on the ware and the cones were melted down. I could remove the bumps with diamond sandpaper but again the glaze looked very dry. All of the other glazes I tried had pinholes -- the two I kept for further experimentation (Wollastonite Clear and Studio White) had the least. I did take G2936B and MC6 Glossy Base liner for a further exploration but they were not fluid enough to see any improvement in pinhole number. Wollastonite Clear and G2936 are almost the same recipe but WC has less silica. 2. Pinholes look like they could very well be from clay contaminates that haven't burned out in bisque. I agree that this is possible. If so, is the best solution for a hobby potter, to use a fluid glaze to flow into the pinholes? The clay composition is Kaolin 25-65% Crystaline Silica - quartz 25-65% Titanium Dioxide <2%. If so, maybe I need to alter the Wollastonite Clear to be more fluid. Any suggestion on the best way to do this without raising the COE or the KNaO? With the Studio White maybe I could use Gillespie Borate instead of the Gerstley Borate to make it more fluid?? 3. I will refire one of the pots with the glaze paste in the pinholes to see if this results in pinhole free ware. Thank you for the suggested firing schedule. I will try it out in the next few days and post the results. Peter: 1. Question on "wollastonite is well sieved, preferably with one of the additives mentioned." Good suggestion but I'm not sure what "additive" you are referring to. I add epsom salts to increase the thixotophy of the glaze but don't add anything else. 2. Regarding contamination: I sieve the glazes twice before I use them so don't think they are being contaminated. I don't have a ball mill but I did unsuccessfully try a mortar and pestle which could become a full time job to just get 500 grams. Cheers. Quote Link to comment Share on other sites More sharing options...
Min Posted February 15 Report Share Posted February 15 (edited) Making the glaze more fluid might work but before trying that I'ld give the other firing schedule a try. Having a longer time leading up to the higher temperature might be enough. edit: if you want to try a more fluid glaze at the same time this might be worth testing. It is a lower expansion than your Wollastonite Clear though. I kept it to materials you already have. ^6 Test White - fluid Ferro Frit 3134 - 34 Silica (325 mesh) -27.5 EPK - 16 Amtalc - 12.5 Nepheline Syenite - 10 total 100 Tin Oxide - 5 Edited February 16 by Min typo and edit Marilyn T 1 Quote Link to comment Share on other sites More sharing options...
Bill Kielb Posted February 16 Report Share Posted February 16 (edited) 20 hours ago, Marilyn T said: I appreciate any ideas you put forth as I have a 100+ dinner set ready to glaze in a white. I bisque at ^4. I am thinking this is bisque to 04 not 4. One thing strikes me in the recipe, boron is fairly high for cone 6. About 0.15 ought to get you cone 6, so this likely begins to melt much earlier. Might be worth an experiment or two reducing the Fritt a bit. Saves on an expensive ingredient and ought to reduce fluidity a bit which might be ideal. Definitely need to test to see if it affects the glaze positively or negatively. Edited February 16 by Bill Kielb Marilyn T 1 Quote Link to comment Share on other sites More sharing options...
Min Posted February 16 Report Share Posted February 16 @Marilyn T, you could shorten the bisque schedule for typical domestic ware pots. Have a look at Steve Davis's schedule for problematic clay. Note the slowing down during carbon (both organic and inorganic) and sulphur burnoffs. (It's behind a paywall but you can access 3 free articles a month) Skip the length candling if your greenware is bone dry, shorten it to an hour or two if not. I usually warm the kiln up to 220F in the evening then shut it off and let the pots sit overnight in the kiln then start it up in the morning. Davis schedule is really slow, but still faster than the Clement one. https://ceramicartsnetwork.org/daily/article/A-Bisque-Firing-Schedule-to-Help-Prevent-Glaze-Faults Marilyn T and Hulk 2 Quote Link to comment Share on other sites More sharing options...
Marilyn T Posted February 16 Author Report Share Posted February 16 Thanks Min and Bill. I'll work on your suggestions over the next week and report back. Bill Kielb 1 Quote Link to comment Share on other sites More sharing options...
PeterH Posted February 16 Report Share Posted February 16 11 hours ago, Marilyn T said: Peter: 1. Question on "wollastonite is well sieved, preferably with one of the additives mentioned." Good suggestion but I'm not sure what "additive" you are referring to. I add epsom salts to increase the thixotophy of the glaze but don't add anything else. 2. Regarding contamination: I sieve the glazes twice before I use them so don't think they are being contaminated. I don't have a ball mill but I did unsuccessfully try a mortar and pestle which could become a full time job to just get 500 grams. What size sieve do you use? Several people - including Min - pre-sieve their wollastonite through an 80-mesh sieve. This thread discussed the use of additives to ease this process, and possibly reduce the size of the larger filtered particles. Quote Link to comment Share on other sites More sharing options...
Hulk Posted February 16 Report Share Posted February 16 Nearly empty kiln, likely that cooling is significantly faster than kiln loaded with ware and kiln furniture (shelves and posts, heavy). As Bill has pointed out, the cooling rate can be slowed (difficult to speed up, however, other than reducing mass). With less mass, heat up could be faster, the second ramp, particularly, if not the last ramp up, which you've capped at 100°/hour... The smaller marks are "healed" over; the bigger pits/holes are the problem, ah! The close-up image, looks like stubborn chunk of off-gassing stuff in the clay, and you're already holding a full hour ~1500°F. With kiln vent running throughout? ...ah, yes, EnviroVent, check. If it is stubborn bits in the clay, long side glance at other batch/source of clay? Quote Link to comment Share on other sites More sharing options...
Marilyn T Posted February 16 Author Report Share Posted February 16 Hulk: I hope it doesn’t mean a change of clay as this is the new clay I changed to after having a delayed crazing issue with Laguna 65. Fingers crossed that my next experiments with fluidity and firing schedule have a happy ending. Hulk 1 Quote Link to comment Share on other sites More sharing options...
Hulk Posted February 16 Report Share Posted February 16 Fingers crossed! Quote Link to comment Share on other sites More sharing options...
Min Posted February 16 Report Share Posted February 16 1 hour ago, Hulk said: Nearly empty kiln, likely that cooling is significantly faster than kiln loaded with ware and kiln furniture (shelves and posts, heavy). I would put furniture in. 1 hour ago, Hulk said: If it is stubborn bits in the clay, long side glance at other batch/source of clay? I think it is the clay. Hoping a more fluid glaze will fill the pinholes but might not be enough. Plates are going to be the litmus test as there isn't gravity to help. 6 hours ago, PeterH said: Several people - including Min - pre-sieve their wollastonite through an 80-mesh sieve. I have a newer bag of wollastonite that isn't as agglomerated since I wrote that I needed to whiz it in a blender, now I just add an extra little bit extra when I'm weighing out the materials to compensate for what is left in the sieve. My bucket top up batches have 985 grams of wollastonite, so I bump this up to 995 when weighing it out. PeterH and Callie Beller Diesel 2 Quote Link to comment Share on other sites More sharing options...
Marilyn T Posted February 16 Author Report Share Posted February 16 Bill: I'm replying to the suggestion to lower the B2O3 because I spent some time this AM thinking about it. I know that a good average is .15 from the work that Katz did, but I think the B2O3 is okay in the Wollastonite Clear recipe as it almost the same recipe as G2926B (less silica) and Toni has explained the .33 level this way: "Just enough to melt to the degree you need. This is different for each recipe (e.g. G2934 only needs 0.12 to have a fluid melt, G2926B needs 0.34). Complex eutectics are involved in every recipe. If lithia or zinc are present less boron is needed." He fires this using the C6DHSC schedule. With the Studio White at 0.15 and without testing: the presence of Zinc seems to indicate it will melt well at cone 5.5. Decided that if I am going to continue experimenting with fluidity, then I should do some melt flow tests. (Toni's flow tester is too expensive for me). I looked at the B2O3 recipes that I previously tested and they range from 0.15 up to 0.34. I selected WC and Studio White because they had less pinholes in all firing schedules. The others had a cross section of B2O3 levels: e.g. Marc's clear, Sue's clear, Glossy Base Glaze Liner, and all the rest of the MC6 base glaze bases in their book, as well as numerous others. Thank you for suggesting this, it made me delve deeper into my understanding of the B2O3 level. Please come back at me with any additional thoughts on this as I haven't ruled out changing the B2O3 level to make it more fluid. Cheers Hulk 1 Quote Link to comment Share on other sites More sharing options...
Bill Kielb Posted February 16 Report Share Posted February 16 16 minutes ago, Marilyn T said: Thank you for suggesting this, it made me delve deeper into my understanding of the B2O3 level. Please come back at me with any additional thoughts on this as I haven't ruled out changing the B2O3 level to make it more fluid. Yeah just a thought really. Not saying it is awful at 0.33, just it likely melts lower than cone 6 (under Katz):which potentially could be causing your holds to not be as effective. If true, reducing it might save on materials as well. Just something easy to try which may indicate reduction in fluidity is something to focus on here. Might not improve things as well. I have used the Katz numbers for several years now as a guide and thus far to good result but my premise is some glazes pinhole worse when more fluid. Firing them higher would be a good example of a glaze behaving counter intuitively as more fluid ought to heal, but in some cases because of the fired surface tension they behave opposite and respond to a drop and hold to heal. So less fluid condition and the glaze can overcome and heal these pinholes. Just an observational thought though. If it works, it also could save on expensive materials. Quote Link to comment Share on other sites More sharing options...
neilestrick Posted February 16 Report Share Posted February 16 I agree regarding the firing schedule. A shorter firing with holds in the proper places could cuts hours off the firing time, and probably reduce firing costs as well. The slow creep to 300F is totally unnecessary, especially if your work is dry. Just run it up to 200F or 220F, hold as long as needed to make sure everything is dry, then go for it. Also, that first ramp can be a lot faster than 60F/hr, like 150F/hr will work just fine. There's no reason to take 2 hours to get to the preheat temp, because the hold is where you're really drying things out. Callie Beller Diesel and Hulk 2 Quote Link to comment Share on other sites More sharing options...
Min Posted February 16 Report Share Posted February 16 Have you tried the Rutile Green base with 5 tin (and no rutile or copper)? Quote Link to comment Share on other sites More sharing options...
Marilyn T Posted February 17 Author Report Share Posted February 17 Yes — the base by itself had pinholes. (No rutile or copper added). Quote Link to comment Share on other sites More sharing options...
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