Chrissa Posted May 22, 2020 Report Share Posted May 22, 2020 Link to comment Share on other sites More sharing options...
Min Posted May 22, 2020 Report Share Posted May 22, 2020 Hi Chrissa and welcome to the forum. What is the black engobe recipe? Link to comment Share on other sites More sharing options...
Chrissa Posted May 22, 2020 Author Report Share Posted May 22, 2020 Hi Min, thank you! Sorry forgot to mentioned the engobe, I've tried Amaco Velour Black, Amaco Jet Black, and Colorobbia HC 669 Black so far. with same results... Link to comment Share on other sites More sharing options...
Min Posted May 22, 2020 Report Share Posted May 22, 2020 Perhaps the black(s) contain enough stains to be making the body impervious to the glaze. I'ld bisque lower and see if that helps or apply them to bisque let it dry thoroughly then try glazing. The gums in the underglazes make getting a heavy enough coat on the underglaze a bit more difficult but worth a try. Hopefully someone else will chime in here who has had this issue and solved it, it's not something I've heard of before. Re pinholes, what is your firing schedule for glaze? Link to comment Share on other sites More sharing options...
Sorcery Posted May 22, 2020 Report Share Posted May 22, 2020 I just watched Chapter 9 with Phil B. At Washington Street Studios. He goes into this very thing, specifically, the difference between an engobe and an underglaze, and what people mean when they say it! And.... How some glaze won't adhere to some underglazes. I would try one coat of underglaze. Or switch the clear. I was thinking this morning, while pondering gutter meteorites, why clears are so ....so.... frustrating. I'd recommend the entire series from WSS too! Sorce Link to comment Share on other sites More sharing options...
Min Posted May 22, 2020 Report Share Posted May 22, 2020 @Sorcery, agreed the Washington Street Studio videos are great! I don't recall them mentioning the problem the OP is having with underglazes and engobes though as I'm reading it. I'm reading that the OP issue is the raw glaze isn't adhering to the underglaze or engobe during the application. @Chrissa, can you confirm this is the issue and not glaze crawling or shivering during the glaze firing? Link to comment Share on other sites More sharing options...
neilestrick Posted May 23, 2020 Report Share Posted May 23, 2020 Chances are it's so saturated with oxides that it's fusing to the point that it's starting to vitrify, preventing the glaze from adhering. Then in the firing it's starting to melt and react with the glaze causing bubbles. Try a different black. Link to comment Share on other sites More sharing options...
Chrissa Posted May 23, 2020 Author Report Share Posted May 23, 2020 Thank you all for your replies! Yes @Min the issue is the raw glaze not adhering to the engobe during the application. I tried bisque firing lower but the results were the same. In addition, this particular clay is an extra strong earthenware clay that is ideal for earthenware tableware and in general I am very satisfied by it, but it needs bisque firing at 1040-1050 Celsius so I cannot go lower.. and unfortunately I can't switch the clear glaze, as this is the only one suitable for this particular clay. @neilestrick Yes, this makes sense. I have tried three different blacks including the black from the same company (colorobbia) and I saw no difference. Thank you @Sorcery for the recommedation I will check them out! I will try the one coat of underglaze (even though I would prefer it bold), thanks for the suggestion! I, too am confused as far as the difference betwqeen engobes and underglazes is concerned I use only engobes/underglazes that come in liquid form (mainly Amaco velvets) and Colorobbia engobes. I will also try applying it to bisque thank you @Min, but usually when I do this with all underglazes the clear glaze is not adhering at all... I will inform you of the results! Here is my firing schedule for glaze: step 1: 80celsius/hour - 250celsius final temperature --- 00.00 hold time step 2: 100c/hour - 450c final temp. - 00.00 hold time step 3: 125c/hour - 700c final temp. - 00.00 hold time step 4: FULL (>999c / hour) - 1040c final temp. - 00.20 hold time Link to comment Share on other sites More sharing options...
Min Posted May 23, 2020 Report Share Posted May 23, 2020 I've used the Amaco Jet Black, it doesn't need to go on heavily to be very black, are you thinning the underglaze down with water before using it or using it straight out the jar? Your glaze firing schedule could be part of the problem with your pinholes. Typically you want to slow the firing down during the last 100C of the firing, not speed it up. I would go at 65C/hr up to 120C, then 200C/hr up to 900C then slow it down to 50C/hr to final temp plus your hold. You might need to decrease the hold time since the slower rise to final temp will be adding more heatwork than your Full (>999/hr) ramp, depending on how fast it actually did increase the temp. Use cones to verify. Link to comment Share on other sites More sharing options...
Chrissa Posted May 23, 2020 Author Report Share Posted May 23, 2020 I use it straight out the jar...yes the jet black is ok with one coat but the colorobbia one needs definitely 2 coats. I will try thinning it! I will also try your adjustments to the firing curve! Thank you so much you 've been very helpful, I will start doing my tests right away! Link to comment Share on other sites More sharing options...
CactusPots Posted May 23, 2020 Report Share Posted May 23, 2020 I have the same problem using multiple glaze layers. Usually I'm trying a dark base glaze under a crawl glaze. Sometimes the base is the same glaze with a colorant. Sometimes it works for me and sometimes the top glaze just flakes off completely. My problem, as I see it, is that the top crawl glaze needs to be pretty thick but without enough water, the bond of the top glaze won't go all the way to the bisqued pot. Glaze over glaze has a lot of variables, so sometimes, it just isn't going to work. Link to comment Share on other sites More sharing options...
Chrissa Posted June 5, 2020 Author Report Share Posted June 5, 2020 Hello again! So, I did my tests: 1. The only solution I found about adhering problems of clear glaze on the black underglazed areas was to bisque fire lower, at 1000C even though the manufacturer suggests higher, and applying a single layer of engobe (it was not so solid as before but I'm ok with it). 2. The firing schedule you suggested @Min helped with the bubbles and the pinholes, they were eliminated, although they still exist to a lesser extend. But in general the fired pieces look better than before so thank you for the suggestion! I fired at 1040C and added a 10 minutes hold time at 1040C. Do you think hold time has anything to do with the bubbling and pinholing? Should I maybe add more hold time or decrease it? I know I should do more tests to answer my questions, but truth is I don't have the luxury of time these days to do more tests and I read so many opinions on the internet that I'm confused, so I hope that someone experienced the same problem might help.. Link to comment Share on other sites More sharing options...
Min Posted June 6, 2020 Report Share Posted June 6, 2020 I'm glad you had some success with changing the firing schedule. 8 hours ago, Chrissa said: Do you think hold time has anything to do with the bubbling and pinholing? Should I maybe add more hold time or decrease it? What are the witness cones showing you reached with the new schedule? Since you are using a commercial glaze it can be more difficult to try and surmise what is causing the bubbles and blisters than with one you've mixed yourself. If the pinholes are better with that slower schedule then my hunch would be a longer soak might help, the other thing you can try is dropping the temperature after the soak by 37C and doing another soak for 15-20 minutes. This is what I do with my glazes which are prone to pinholes. The extra soak at the lower temperature adds a bit of heatwork but not much. Link to comment Share on other sites More sharing options...
Babs Posted June 6, 2020 Report Share Posted June 6, 2020 Glad you are claring up issues. Just be clear reunderglaze and engobe.. Not the same species imo. Link to comment Share on other sites More sharing options...
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