Marilyn T

Thanks for your reassuring answer Bill. ^6 self-supporting cone fired to 4 o'clock (optimum outcome was 5 o'clock). The total firing was 14:16. (Digitalfire was 14:20 so the total time was in line and the temps at the ramp changes also correlated). The only anomaly was the rate of 120 instead of 108 so I was a little concerned that the controller was not working properly, however if I don't get an error code then I'll assume it is within its acceptable speed range.

I programmed a C6DHSC program into my Skutt 1018 and the rates matched except in Ramp 3 which I programmed in for a rate of 108. The reading when I pushed the "5" key, which shows the programmed rate of rise per hour showed 120. I pushed the review button and it showed a rate of 108 in that segment. The 120 reading showed all the way from 2100 to 2200 and into the 10 minute hold. The rest of the rate readings are in line. This is the first firing since I changed the relays, elements and thermocouple. The previous ramp 2 had a rate of 350 (in my kiln 344). My amp reading reads 20 for both; This is not my normal firing schedule so I don't have a comparison from an old firing. My guess is that the kiln couldn't cool down from 344 to 108 quick enough but that doesn't explain why the rate didn't decrease over the 2100 - 2200 range. Any idea why this happened? Thanks.

Thanks for your reply Callie. I do like the clay but a defect free surface tops that. My current testing is to establish the best possible results for the Laguna 613 dinnerware set before committing to either refiring it to get a smooth white surface, or to glazing it with Rutile Green or Variegated Blue and using it in my own kitchen. I will add your suggestions to my list of firings as I continue to add to my knowledge base. With regard to the M390 glaze, I accidentally calcined the Ravenscrag instead of roasting it -- but from what I read it shouldn't affect the glaze -- it broke up fairly easily. Thanks for the heads up on the temporary change in texture of the M390. I did find it coarser than the Laguna 613 so hope it is smooth enough to allow rims to be unglazed -- if that is the chosen design. I purchased the box of M390 this month, so not sure if it is a minus 80 mesh sieve batch or not. I'll see what my absorbency tests reveal. Thanks again for generously sharing your ideas.

I have attempted to contact Laguna twice before about a different clay (Laguna 65) and received a reply that they were forwarding my question to someone else -- never heard back. So I probably won't contact them again. I recently purchased a box of M390 and threw some test bowls. I am including them in my next series of tests to see what the results are -- figured if Tony had success on M390 with the glaze G 3806 that I could use that as a comparison for the results on the Laguna 613 and go from there. He uses the C6DHSC firing schedule so if I don't have good results on the M390 it will indicate that something else is going on that I haven't thought about. As part of covering all the bases I'm bisquing a few bowls at ^02 for comparison. Going to give the Laguna clay about another 2 weeks to reveal its mysteries then abandon ship (I only have 6 bags left). -- 6 more glaze firings to go (the golf course is calling!) The good thing is that I am really good at throwing test bowls now.

Update # 2 for anyone following this thread: Results on Wollastonite Clear adapted glaze. Application: Varying the SG from 1.40 - 1.46 did not eliminate or make the problem better. I like the look of the lower SG and it has the added advantage of making it easier for the gases to escape. The addition of epsom salts to increase the thixotropy also didn't result in an improvement. Defect: more pinholes but smaller. (see photo 2X). I put India ink on the ware and viewed it through a jewellers loupe, because at first glance it didn't look too bad (no glasses; bad lighting) Causes of defect: (my theory) gases from body have not been fully expelled due to early melting of glaze which seals the surface (starts at 1800F); clay vitrifies >2150 before all the feldspars are expelled (2044-2192). oversupply of Calcium which has high surface tension (fluxes at 2012) and results in a glaze that is not fluid enough to fill the pinholes. Next steps: 1. try a glaze that has later melters and fluxes which lower surface tension. I will be trying the Studio White adapted recipe that Min sent me, as well as a similar recipe G3806 C and 3806 E from Digitalfire. They have lower B2O3 (0.10 to 0.15) and use Zinc oxide. (Zn limit 0. - 0.2 and the recipes call for 0.12 - 0.14 so shouldn't cause pinholes). All comments/suggestions/recipes welcome. Next update in a couple of weeks when I've finished testing the Studio white glaze.

Update for anyone following this thread: I have now finished my firing schedule experiments and didn't come up with a schedule that worked well. So far there are two possible solutions: (1) refire and (2) only glaze one side (to allow gases to escape from the unglazed side). I will use two firing schedules: my original firing schedule (the one that gave good results on Rutile Green, and Variegated Blue) and Min's suggested firing schedule which gave a good results for Studio White for the next set of experiments -- thickness of application and adjusted glaze chemistry. The results for the over 20+ firings are: peak temperature: the best results were in the range 2165-2190. I used both the 108 ramp and 27 degree ramp. Worse results were experienced going higher or lower than this range. soaks at peak: increasing the length of a hold from 2190: 5 - 30 minutes ; 2165: 18 to 30; and 2133: 30 to 75 didn't improve the results. slow cool rate: tried 500, 125 and 60 and got worse results the slower it went. The VB developed blisters with the extended hold. drop and hold soaks: soaks in the range 1900 to 2090 (going down by 10 degrees) didn't make any difference in the results for the Wollastonite Clear or Studio White. The only success on the VB and RG were in the original firing schedule. Next predicted update in one month.

Thanks Min. I'll do a full cone 6 fire on a small bowl as soon as I bisque some test bowls. I'll also start throwing some small plates to use as testers. What is it about the Studio White that you don't care for it as much as Wollastonite Clear? I only have about 7 L of Gerstley Borate left and was not sure how Gillespie borate would affect the glaze so thought Wollastonite clear would be better in the long run. I chose Studio White, not only because it was a proven glaze, but due to the Silica being the only common material and I therefore thought it would be easier to test the firing schedule as the source of the pinholes. It also has a high LOI compared to the Wollastonite clear so thought I could eliminate the gas from the glaze materials as being the source of the pinholes if they both performed the same (i.e. equal # of pinholes). I'm looking forward to trying out your reworked recipes and appreciate the time you are donating to help me work toward a solution for this problem.

What happened when you fired the Wollastonite Clear to a full cone 6, was the clay bloating or had a higher absorption? Haven't done this yet but the refire didn't show any signs of bloating. Previous tests with other glazes at higher temperature (Glossy Base Glaze Liner and G2936B) also didn't show signs of bloating, but I did get warping on the 25 hour NC firing. At ^7 you had increased absorption, correct? Yes it went from 0% at ^6 to 6.6 at ^7. I can alter your recipe to do this if you want to give it a go. It's worth a try. Thanks I'd appreciate it. Do you have Ferro 3110? I can get some. Presently I have Ferro Frit 3134, 3124, 3195 and 3249. Do you have the recipe for the Studio White? Minspar 200. 47.378 Silica 20.247 Gerstley Borate 13.875 Whiting 8.428 Dolomite 6.064 Zinc oxide 4.008 Bentonite 2.0 (Tin Oxide 5.0) Have you noticed if different thicknesses of the Wollastonite Clear results in more or less pinholes/blisters? I haven't experimented with different SG's yet with this glaze. Tony recommended 1.4 for the G2936B so I kept that consistent for the white glazes. The FAW (1.47) VB (1.43) and RG (1.43) have higher SG. The commercial clear has a SG of 1.45). There hasn't been more pinholes in the bottom of the bowls (where it might be thicker). Most of the original testing (with GBGL and G2936B) had pinholes clustered outside near the elements. At some point in testing that switched to none on the outside and some on the inside. Now it doesn't seem to have a pattern. I also think the pinhole issue is from the claybody but understand you are invested in using this body for now so it looks like glaze formula + firing + glaze fluidity + glaze thickness are all going to play into trying to overcome it. I agree. Good learning experience but it would be nice to have a final product!!

Min: your suggested firing schedule resulted in an improvement. I now have two without pinholes or dimples; one with pinholes; and the rest with dimples. Thank you. # 1 Original firing schedule where the Variegated Blue # 2 Firing schedule where the refire and studio white had no defects and Rutile Green had no defects but the rest did was: ^5 SS bent to shelf; ^6 slight bend. = 5.5 100. 220. 30 100 220. 30 350. 2000. 0 350 2000. 0 108. 2190. 5 108 2165. 18 9999. 2090. 10 9999. 2085. 12 500. 1900. 10 500. 1900. 10 125. 1400. 0 125. 1400. off The results of the glazes for Firing # 2 on 1 lb. bowls were: Wollastonite Clear -- 1 pinhole outside 3 on rim Studio White -- no defects Folk Art White -- dimples Variegated Blue -- dimples Rutile Green -- dimples Refire -- no defects Test Glaze on test tile -- four pinholes on side next to element. I also fired a commericial glaze (Amaco HF-9) at cone 5 + 5 minutes but it had dimples and one large pinhole (^5 bent to shelf). Two potters on instagram fire this clay at 5+ and seem to have good results with commercial glazes. One uses Amaco Honey Flux (which is a fluid glaze). Amaco says some of their glazes shiver on this clay. I welcome all suggestions that might result in a defect free firing schedule for this clay. Neil: I will try your suggestions on shortening the bisque schedule in my next endeavour. Right now I have over a 100 pieces bisqued using the schedule I outlined above, so will concentrate on the glaze firing schedule with the already bisqued pieces. Peter: I don't think it is the wollastonite in the glaze that is the problem (one of the glazes has Wollastonite and the other Whiting), however there may be Wollastonite in the body. I sieved the clay slurry and did find some small white chunks in it that didn't go through the sieve. They may be silica or feldspar?? Photo attached. On one of the bowls that I glazed in a commercial clear, there was a white chunk on the outside, under the glaze that was the same as the one on the screen. Bill: Hopefully the problem will be solved with the firing schedule, so I won't have to change the glaze fluidity (not my strength). I'm thinking your suggestion of multiple holds may be part of the solution. Hulk: Thanks for your suggestions. I still hope to find a solution without changing the clay.

Yes — the base by itself had pinholes. (No rutile or copper added).

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: 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.

Thanks Min and Bill. I'll work on your suggestions over the next week and report back.

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.

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.