dhPotter Posted November 15, 2016 Report Share Posted November 15, 2016 Nice plate except for the big zit on the rim. What causes this? It started happening when I began spraying glazes. Just 1 or 2 per pot but not all pots and it is not the same glaze doing this crater. Link to comment Share on other sites More sharing options...
neilestrick Posted November 15, 2016 Report Share Posted November 15, 2016 Can you post a closer pic of the zit? Link to comment Share on other sites More sharing options...
Roberta12 Posted November 15, 2016 Report Share Posted November 15, 2016 I guess I always thought that was either something coming out of the clay (big piece of something) or a bubble. I will be interested to see what the more experienced say! Roberta Link to comment Share on other sites More sharing options...
Mark C. Posted November 15, 2016 Report Share Posted November 15, 2016 This glaze looks like a rutile/iron comb glaze and if thats a yes than pitting can occur pretty easy. It can be the glaze -atmoshere-dust-clay impurities . The cures are clean ware-more hold times -and for me the right amount of reduction-for you a little more hold time in an electric. If this glaze has rutile and iron as colorants this may always be an issue on and off.Stoneware clay can make it worse. Link to comment Share on other sites More sharing options...
dhPotter Posted November 15, 2016 Author Report Share Posted November 15, 2016 Here ya go Neil... The rim is 2 matte glazes. Strontium Crystal Matte Warm with a Matte Iron Red glaze over. This is stoneware. Have seen this pitting when wiping the bisque with a damp sponge or just dusting with a dry rag. Matte Iron Red Nepheline Syenite A270...... 13.20 Silica...................... 21.90 Calcium Carbonate........... 21.90 EP Kaolin................... 30.80 Talc........................ 4.40 Strontium Carbonate......... 2.00 Tricalcium Phosphate........ 5.20 Lithium Carbonate........... 0.50 Iron Oxide Red.............. 12.20 Copper Carbonate............ 0.90 Bentonite................... 2.00 ========= 115.00 Link to comment Share on other sites More sharing options...
neilestrick Posted November 15, 2016 Report Share Posted November 15, 2016 What cone are you glaze firing to? What type of firing? What clay body? What cone do you bisque to? You said several different glaze combos are doing it? Is there a constant in any of them? Link to comment Share on other sites More sharing options...
Babs Posted November 15, 2016 Report Share Posted November 15, 2016 Have seen this pitting when wiping the bisque with a damp sponge or just dusting with a dry rag. SO the pitting is occurring after a bisque ? Link to comment Share on other sites More sharing options...
Mark C. Posted November 15, 2016 Report Share Posted November 15, 2016 Your glaze is prone to this as it has Strontium Carbonate in it as well as Lithium Carbonate ​Glazes with barium or Strotium are problematic with pitting Seen this as a woe for 40 years. The matt is also a factor. For me its worse with stoneware but it happens on my porcelain. I have used a few of these type of glazes since the 70s and pitting is part of the deal If you ever cure this altogether we want to know about how you did it Link to comment Share on other sites More sharing options...
dhPotter Posted November 15, 2016 Author Report Share Posted November 15, 2016 Mark you are a trip!!! What cone are you glaze firing to? 6 What type of firing? electric oxidation What clay body? stoneware Laguna wc 607, #55 What cone do you bisque to? 04 You said several different glaze combos are doing it? Is there a constant in any of them? It may be the lithium, had another pitting on a mug in the same firing, but the lithium carb is only 1.2% and magnesium carb 2.4%. Was also thinking it could be the whiting, but this other mug that pitted uses wollastonite. Link to comment Share on other sites More sharing options...
oldlady Posted November 15, 2016 Report Share Posted November 15, 2016 that is ugly, john. sorry it is happening. Link to comment Share on other sites More sharing options...
Mark C. Posted November 15, 2016 Report Share Posted November 15, 2016 I'll never comment on area I have no experience in trust me on that. And that is more and more as I get older. One of my mentees I worked with for two years used your Strontium Crystal Matte Warm in her production line for many years(cone6) . I wish I had a dollar for every time these barium or Strontium glaze had flaws. I just did kiln load last week in another cone range and pitting is the enemy of these glazes-They are also all matts. Link to comment Share on other sites More sharing options...
glazenerd Posted November 16, 2016 Report Share Posted November 16, 2016 The fact that only one or two zits appear, makes me suspect of the cause. Secondly, a very large zit to boot: also somewhat of an abnormality of itself. So I will add this thread to my stoneware "watch" list. As stated prior: I am seeing large chunks of white debris in the samples I have been looking at (microscope). At first I thought they were soapstone, but now I am leaning to large particle sizes of feldspar minerals. If that holds to be true: it explains the small number of sites, and the size of the crater. As seen in the 200x close up. (The large dark/black specks are 35M, as a point of reference.) Nerd Link to comment Share on other sites More sharing options...
neilestrick Posted November 16, 2016 Report Share Posted November 16, 2016 When I first saw the small photo I was leaning toward it being something coming out of the clay. The big photo makes me think it's the glaze, though. Try firing a little bit cooler with a hold time. Maybe a little soak will fix the problem. Nerd, what's that program you use with the soak to cone 6? Link to comment Share on other sites More sharing options...
glazenerd Posted November 16, 2016 Report Share Posted November 16, 2016 Neil... sure- do I get to throw in clay lessons to boot? (your normal ramp) to 2050F ( the temp when meta-kaolin forms spinel- the basis of mullite in a stoneware body). Fast ramp above this temp means less formation of mullite, and delays peak off gassing until the top of the ramp cycle. (less mullite also means less vitrification) 140F (from 2050) to 2190F with an extended hold. (or) 2230F with a short hold. Potassium is normally the flux of choice in stoneware bodies, primarily because many of the fire clays, and ball clays used in stoneware have a fair percentage of Ka to begin with. 2050F to 2180F is when potassium is at its most effective melt temp, which also means its peak off-gassing temp. Porcelain and stoneware are porous up to 2050F, when the effective melt of the potassium begins to close the pores as spinel is developing. Blowing past this key temp (2050) too quickly also means you are sealing the face of the clay directly exposed to the kiln temp, but the inner body is still ejecting gas. The key piece of evidence of creating this "trapped gas" scenario is much larger craters in the glaze: because the gas is trapped until enough pressure is built up to forcibly eject it through the clays surface, and through the glaze. Potassium and sodium are in a gaseous state from 2050F,and up... (starts before that, but begins to hit its peak) 2190F with extended hold ( 15-20 min. for smaller kilns, and up to 45 minutes for large kilns) 2230F with shorter hold ( 5-10 minutes for smaller kilns and up to 30 minutes for large kilns) < the one I use. Not had any pinholes in over 2 years using this ramp cycle. Nerd Link to comment Share on other sites More sharing options...
dhPotter Posted November 16, 2016 Author Report Share Posted November 16, 2016 Excellent information! MarkC you are a good trip! Thanks so much for your input! Will adjust the firing schedule. Maybe can report results in about a week. BTW I have another piece, 2 matte glazes overlapping, with a bigger crater. It is a green and blue matte. The biggest craters are on flat surfaces, this plate and a laid out platter. Kiln - Olympic 2327 HE - about 7 cu ftGlaze cone 6...Segment Rate F*/HR Temp Hold 1 100 230 30 2 400 1975 0 3 100 2185^ 15 4 9999 1700 0 5 50 1600 60 6 50 1500 0 GlazeNerd are you saying when the kiln hits the 1975* mark to slow down to say 50* per hour till hit the 2185* and hold? Link to comment Share on other sites More sharing options...
glazenerd Posted November 16, 2016 Report Share Posted November 16, 2016 DH: >your normal ramp> to 2050F <140F to 2190F with hold (pending kiln size) or to 2230F with a much shorter hold. 2050F to 2180F is the most critical period for stoneware clay maturity. The pottery biz has gotten in the habit of firing to mature glaze, which has the side of effect of not maturing the clay. If you fire to mature the clay, the glaze will automatically follow suit. My firing schedule is : 400F to 2050F 140F to 2228F with 5 min hold.... bends a cone 6 perfectly. Not had a pinhole in 2 years, except for some of my "extreme" experimental glazes. Nerd ** pending kiln amperage (wattage) some adjustments can be made. Some kilns would struggle to 2050F @ 400F. Link to comment Share on other sites More sharing options...
dhPotter Posted December 6, 2016 Author Report Share Posted December 6, 2016 GlazeNerd, Changing the schedule to what you suggested worked. Not many and only very minor craters. Not near as many or as severe as before. The pic on the left has an iny and an outy crater. Thank You Link to comment Share on other sites More sharing options...
glazenerd Posted December 7, 2016 Report Share Posted December 7, 2016 DH: Add 10 minutes to your peak hold--and you should be home. OR ramp 10 degrees an hour slower on the high end cycle. It is the clay off gassing; so the particular blend you are using has a higher than normal amount of potassium flux. Which is actually a good thing ( pin holes aside) because the vitrification levels will be much higher. You should also see a big difference in "weeping" issues. Now you are maturing the clay, not just the glaze. Nerd Try 130F from 2050 to your hold temp. "the inny and outty" says you are right at the border. One was healing ( the inny) and the "outty" says a bit more time. Link to comment Share on other sites More sharing options...
dhPotter Posted December 7, 2016 Author Report Share Posted December 7, 2016 Very Good, Nerd. Thank You. Link to comment Share on other sites More sharing options...
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