PottaFella Posted May 12, 2014 Report Share Posted May 12, 2014 Hi experts. The attached images show latest glaze which has a matt finish. I like its colour and orange-peel surface texture but it is affected by pinholes – not so nice. I should say I am not bothered by the largish pinholes in the narrow pale grey blotched band near the pot's base. It is the dozens of tiny pinholes over the main outside and inside surfaces which are a problem especially if I want to make serving pots. I don't think it is caused by dust because the pinholing is all over, in and out, and same amount both on recently bisqued pieces and some which have been hanging around a while in the 'studio'. I probably put on a thicker than usual layer of glaze as it was quite thick & creamy in the bucket. The pots may not have been entirely dry from the glaze when they went into the kiln (I was in a rush). Clay body is a Valentine's toasted stoneware which I would say has a little texture to it (but definitely not heavily grogged). The glaze is : Soda Feldspar 70.0 China Clay 13.0 Dolomite 5.0 Whiting 4.0 Zinc oxide 5.0 Quartz 3.0 Plus a smidgen of cobalt and nickel for the grey/blue colouring. Some Calcium Chloride was added to glaze to aid suspension. Bisque firing was to 1000c. Glaze firing was electric 0-600c 100 per hour, 600–1230c @ 150c and 30min soak. Then straight down which takes about 16 hours. Any suggestions folks? Link to comment Share on other sites More sharing options...
Guest JBaymore Posted May 12, 2014 Report Share Posted May 12, 2014 Pinholing is probably 90 percent caused by bisque firing and clay body issues... not the glaze or the glaze firing. Please describe in more detail your bisque firing process....... what is the cone that is down? What are the rates/ramps. etc. Do you have a local pickup (downdraft) VENT on the kiln? What types of forms does the pinholing happen more on (if is tends to do so)? Does it happen evenly on pieces that were stacked in all areas of the bisque kiln... or more on pieces from some locations? Worse inside or out, or pretty even? Any other glazes doing it? Does the same glaze do it on a different body? best, .......................john Link to comment Share on other sites More sharing options...
PottaFella Posted May 12, 2014 Author Report Share Posted May 12, 2014 Dear Doctor. Bisque process is 0–600c @ 100c per hour, then full power up to 1,000c. It is a very efficient kiln so that is perhaps 200c per hour but I would have to time another firing to find out. I am sorry I don't use cones (I keep meaning to). Vent: just small open spy hole near top. Bottom vent is always closed. Pinholing is perhaps slightly less prevalent on open bowls, more prevalent on tall shapes, vases, jugs. Not certain about the areas of kiln from bisque, but every item from this glaze firing is affected, so if pushed I'd say all bisque kiln areas equally bad. Inside and outside of pots evenly and equally affected. I have had a pinholing using the same base with different colourants/oxides, but never so bad. In other firings it has affected another clay as well – a Valentines white Earthstone which has some fine white grog in it. My firing programs are always the same. Link to comment Share on other sites More sharing options...
Wyndham Posted May 13, 2014 Report Share Posted May 13, 2014 The rim of the piece has melted more(usually does) and healed over, where as the rest of the body looks under fired. Maybe a soak at the end of glaze firing for 10 or 15 min might help along with a slower bisk to 04 Wyndham Link to comment Share on other sites More sharing options...
neilestrick Posted May 13, 2014 Report Share Posted May 13, 2014 I agree with Wyndham- that glaze is under fired. It's really high in alumina and stiff to the point that it's not smoothing over completely. It's also very high in soda. Combined with being under fired, it's probably not a very durable glaze at all. Look for matte glazes that go matte because of the makeup of the glass, rather than simply being under fired. Magnesium matte glazes are a great way to go. They are durable and smooth, you can make them runny but still matte, and you can control how matte they are by how slow you cool the kiln. Link to comment Share on other sites More sharing options...
oldlady Posted May 13, 2014 Report Share Posted May 13, 2014 does wetting the bisque before glazing help? I don't usually dip pots anymore but recently, when I dipped dry ones into a bucket of glaze, they holed immediately and looked just like the ones pictured. I will be washing them all and trying again, this time dampening the bisque before glazing. Link to comment Share on other sites More sharing options...
PottaFella Posted May 14, 2014 Author Report Share Posted May 14, 2014 The rim of the piece has melted more(usually does) and healed over, where as the rest of the body looks under fired. Maybe a soak at the end of glaze firing for 10 or 15 min might help along with a slower bisk to 04 Wyndham I've got a 30 min soak at moment, but I could make that longer or increase the top temperature by maybe 10 degrees C? Link to comment Share on other sites More sharing options...
PottaFella Posted May 14, 2014 Author Report Share Posted May 14, 2014 I agree with Wyndham- that glaze is under fired. It's really high in alumina and stiff to the point that it's not smoothing over completely. It's also very high in soda. Combined with being under fired, it's probably not a very durable glaze at all. Look for matte glazes that go matte because of the makeup of the glass, rather than simply being under fired. Magnesium matte glazes are a great way to go. They are durable and smooth, you can make them runny but still matte, and you can control how matte they are by how slow you cool the kiln. Thanks Neil. Underfiring seems to be my speciality. I've got quite lot of this glaze mixed up! Do you think maybe to increase the glaze firing top temp by 10 C to maybe 1240 C would help? Looking back at other pots using similar glaze on other clays, they don't seem to have had such problems with pinholes, so maybe it is also something to do with this 'toasted' clay I've used? Could it have too much 'stuff' in it that makes pinholes? Also can you poss tell me, does a thicker glaze equal more pinholes? Link to comment Share on other sites More sharing options...
Guest JBaymore Posted May 14, 2014 Report Share Posted May 14, 2014 I ran the glaze as best I can (Soda Spar and China Clay are way too 'generic' for much accuracy in calc) and it came out within limits for the general end point temp you are running (extrapolated to a cone). As Neil said...... it is high in soda (but in limits)... but that could be because of the generic "soda spar" used in the calc. And the alumina is just under the top end fior alumina, but within limits. No active vent on the bisque kiln (no good airflow) and the fact that it happens heavily on this clay and not on most others, that it happens more on the lower (contained to bisque air flow) parts of the forms, all make me suspect bisque issues. Yes the glaze will be "stiff" due to the high alumina..... which if pinholes are forming... will tend to prevent their healing over for SURE. So it is very much possible that a bit higher cone end point will allow them to heal over. But it would be interesting to see if that problem can be fixed by not having them develop in the first place....... which MAY be coming from the lack of organics being burned out at the right time in the bisque, and continued outgassing of some other reactions. And you should really be using cones in the load to control endpoint ......... not just a temperature on a thermocouple. The thermal lag between the thermocouple ptrobe outin the open and the considion of the wares back in the load can vary a lot based upon the stacking density. best, .........................john Link to comment Share on other sites More sharing options...
PottaFella Posted May 14, 2014 Author Report Share Posted May 14, 2014 I ran the glaze as best I can (Soda Spar and China Clay are way too 'generic' for much accuracy in calc) and it came out within limits for the general end point temp you are running (extrapolated to a cone). As Neil said...... it is high in soda (but in limits)... but that could be because of the generic "soda spar" used in the calc. And the alumina is just under the top end fior alumina, but within limits. No active vent on the bisque kiln (no good airflow) and the fact that it happens heavily on this clay and not on most others, that it happens more on the lower (contained to bisque air flow) parts of the forms, all make me suspect bisque issues. Yes the glaze will be "stiff" due to the high alumina..... which if pinholes are forming... will tend to prevent their healing over for SURE. So it is very much possible that a bit higher cone end point will allow them to heal over. But it would be interesting to see if that problem can be fixed by not having them develop in the first place....... which MAY be coming from the lack of organics being burned out at the right time in the bisque, and continued outgassing of some other reactions. And you should really be using cones in the load to control endpoint ......... not just a temperature on a thermocouple. The thermal lag between the thermocouple ptrobe outin the open and the considion of the wares back in the load can vary a lot based upon the stacking density. best, .........................john Thank you very much indeed for your advice and research John, that is very kind. I am ordering some 5, 6, 7 cones, do you advise using cones for the bisque firing too? I'm thinking to try a higher bisque temp 1050 C (rather than 1000 C). And then a slightly higher glaze firing of 1240 C (rather than 1230 C). I will keep to a 30 min soak I think for now. How does that sound? Can you possibly advise, to attempt to 'fix' my pin-holed pots, I'm thinking to rub some sticky glaze into the holes and fire again at 1230 C again or would you suggest higher or lower? I read somewhere that second firings don't need so much heat? PS. I have just read an article on about.com of how bisque firing should be very gradual with an overnight warm-up, I have not been doing this. Do you think a much slower increase in bisque temperature would make a difference to how the 'organics' you mention are being burned off? Link to comment Share on other sites More sharing options...
Marcia Selsor Posted May 14, 2014 Report Share Posted May 14, 2014 It looks a little thick on the application. As Old Lady suggested, wet the piece before glazing so the glaze is not absorbed too fast or too much.If you glazed the inside first, which looks like the case, the glaze is on much thicker than the outside. Yet, the outside still pinholes. Soaking at the final temperature would help the melt. And a higher bisque may help to eliminate whatever may be causing the pin holing. How high do you bisque? What is in your clay body? Marcia Link to comment Share on other sites More sharing options...
PottaFella Posted May 14, 2014 Author Report Share Posted May 14, 2014 It looks a little thick on the application. As Old Lady suggested, wet the piece before glazing so the glaze is not absorbed too fast or too much.If you glazed the inside first, which looks like the case, the glaze is on much thicker than the outside. Yet, the outside still pinholes. Soaking at the final temperature would help the melt. And a higher bisque may help to eliminate whatever may be causing the pin holing. How high do you bisque? What is in your clay body? Marcia Thank you Marcia. The pieces were completely dry when I glazed them. Yes, the glaze is definitely a bit thick. You are correct I did glaze inside first. The bisque is 100 C per hour 0–600 C, then straight to 1000 C. The clay body is Valentine's 'Toasted' Stoneware which is slightly textured, not really grogged. si oly Link to comment Share on other sites More sharing options...
clay lover Posted May 14, 2014 Report Share Posted May 14, 2014 You might contact the clay manufacturer to see what bisque the recommend for this clay. and, Yes, use cones in the bisque until you figure out what is really going on temp wise, in your kiln. The temp you think you fire to is not the degree you get, often. Heat work is what it's about, and the cones are the only think I know of that tell what really happened in the kiln. And you need to know that to problem solve. When you change things, change only 1 thing at a time, the soak or the top temp but not both , or you won't know which change did what. Link to comment Share on other sites More sharing options...
Guest JBaymore Posted May 14, 2014 Report Share Posted May 14, 2014 When you change things, change only 1 thing at a time, the soak or the top temp but not both , or you won't know which change did what. BINGO! Basic scientific method. Too many people do the "shotgun approach" to solve issues........ fire enough pellets and you are bound to hit something . But you never improve your aim that way. best, .......................john Link to comment Share on other sites More sharing options...
PottaFella Posted May 15, 2014 Author Report Share Posted May 15, 2014 If testing changes like this, does one just fire a single piece, leaving the rest of the kiln empty? I guess that changes the kiln environment a bit? Link to comment Share on other sites More sharing options...
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