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Thrown_In_Stone

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  1. Pinholes After Decals Stoneware

    The Bisque is a medium speed cone 04 on smooth white stoneware. Potters Choice glazes brushed on fired to cone 6 medium speed. The glaze is perfect after the glaze firing. The only defect is after the decal fire for which the firing schedule is 250ºC/hour up to 830ºC with a 15 minute soak, natural cooling.
  2. Hi everyone,. I usually fire little decals with my logo on to the bottom of every ceramic item I produce. I have just moved from Earthenware to Stoneware after having had excellent results with Earthernware decals but after doing the decal firing on stoneware I am getting pinholes and bubbling all over the stoneware glazes (not just on the decal but everywhere). I have followed the firing schedule my decal supplier has advised which is 250ºC/hour up to 830ºC with a 15 minute soak, natural cooling. The colour of the glaze also changes slightly. The stoneware is cone 6. And the glaze is mostly Potter's choice to ^6. There are also other brands of glaze which is affected. Any Advice? Would you increase the temp of the decal firing in the hopes that it will heal over the pinholes or reduce the decal temp in the hopes that it will stop the glaze melting so much. (And what temperature would you recommend). Thanks for any help you can give.
  3. Glaze Disaster

    Thanks for your comments. I actually like the vase too, In fact it's in my kitchen filled with water to test if it leaks (being earthenware), If it doesn't I'll be selling it or at least using it for myself, looks like it should have come out like that! I should note that I wasn't best pleased as this is my brand new Skutt kiln; 3rd firing ever (fist glaze firing). However I know this isn't the kilns fault as it did it in my old one once, not quite the same glaze combo but almost the same. Thankfully not too much damage to the kiln, most of the glaze came off the batt wash with only minor grinding needed. Also ran off the shelf onto the bottom slab which after removal left a 1cm deep hole in one of the bricks *SOB* . Though I'm extremely thankful that it wasn't worse, as one of the splashes was 15mm away from the bottom element groove (how lucky am I?). The glaze in the mugs is not the white, it's transparent but almost identical base glaze. I'll try find the recipe for the blue and post it later.
  4. Glaze Disaster

    Well, I seem to have found one of my famous explosive glaze combinations again. I have 2 glazes (Blue and white) The blue is fabulous, it's never let me down but I tried putting a white over the top of it this time and OH MY DEAR GOODNESS. I attach a picture of the the result in in the kiln and one of the pots. The glaze doesn't seem to have run off the pot, more fallen off; I just don't know at what point during the firing though. The recipe for the white is: Calcium Borate Frit: 39 Soda Feldspar: 27 Whiting: 5 China Clay: 6 Quartz: 23 Zirconium Silicate: 10 Cone 04. Any ideas? Is there ingredient that may be in the Blue glaze that the zirconium it's reacting badly with?
  5. Hello, I have just bought a new Skutt kiln and wondered what setting I should use for bisque. I'd like to use the cone firing mode but should I use the slow setting with no pre-heat, or the medium with a 1hour preheat (which is 60ºF per hour up to 180 then hold fir 1 hour). My greenware is dry. Thanks for any advise you can offer.
  6. Rutile - Earthenware

    I think with the right glaze and flux, you should be able to get some interesting effects. The glaze, I would think would need to be a bit fluid and be used as a capping over a more stable base glaze. You might test by glazing a piece with a know glaze then making up a wash with rutile and a frit like 3269 or 3110. Do a line test rutile 25 frit 75 and in 10% increments go to 75 rutile/25 frit. Give it a try. Hope this helps Wyndham Thanks for your advise, I'll give it a try. I note you say 3110 which is a high alkaline frit and is commonly used in crackle glazes. If I do a wash with this is it likely to cause crazing of the below glaze? Cheers, Gary
  7. Rutile - Earthenware

    Evening all, I was just wondering, does Rutile promote mottling and streaking or other interesting effects in Earthernware glazes approx. cone 04? Or does this just happen in stoneware. Many thanks.
  8. Glaze Problem on slipcast items

    Thanks for your input. No, there's no glaze chips on the shelf at the end of the firing. I put another kiln load on yesterday, I'll see how this one goes. My normal firing schedule is 3:30 to 500ºc then full power to 1060ºc. The firing normally takes around 9 hours. The firing from last night I had it on a ramp of 4:45 to 500ºc then full power to 1060ºc with a hold of 2min. I increased the first ramp to ensure it was slower whilst any remaining water was pushed out from the glaze incase this was dislodging it. I guess for what you describe you mean slow the top end. On my next firing should I slow it down by an extra hour?, ie something like this: 5:00 to 500ºc then 5:00 1060ºc? I've never used a ramp at the top end, I hope the kiln can cope with it. It seems to go slowly when it gets above about 900. I don't want it to suddenly switch off with an error that it can't heat fast enough. I guess I'll not know until I try.
  9. Glaze Problem on slipcast items

    What kind of soak time would you put in? Would you reduce the final temp to compensate slightly?
  10. Glaze Problem on slipcast items

    The epson salts are in the glaze not the clay.
  11. Glaze Problem on slipcast items

    This makes me think there may be something soluble in either the slip or glaze that is coming to the surface. Maybe try a different water supply with a small batch, bottled water, etc. Can epsom salts affect anything?
  12. Glaze Problem on slipcast items

    Several questions spring to mind here. How are you glazing? Are you glazing the inside and outside separately and in what order. If so what is the drying time between the inside and the outside? Are you brushing on the glaze? Are you spraying the glaze? I ask these questions as it seems to me that you may have a problem with crawling (glaze pulling away from the surface during the firing). I have experienced this when dipping glazes on thinly constructed work. The bisque becomes saturated when more glaze has been added to a surface. Bubbles or blisters appear on the unfired glaze during drying. These can be smoothed back by rubbing. However in the majority of cases the glaze crawls during the firing because of the lack of adhesion of the glaze to the surface. With brush on glazes this can also occur when subsequent layers can lift off the first layer be it ever so slightly. With sprayed glazes this can also happen when too thick a layer is applied in one hit rather than several thinner layers. One way or another the bisque can becomes saturated. Saturated areas do not have good adhesion with the glaze. With reference to your statement "about 70% of the problem pots have the problem on the side facing the elements" it may be that the side facing the elements initially heats up at a more rapid rate than the rest of the pot and trapped moisture causes further problems with poor glaze adhesion. I note also that your bisque temperature is higher than your glaze temperature. Could this be contributing to the poor adhesion? Regards Johanna Thanks for your reply. Some good things for me to test out here. At the moment I'm dipping them in one. Do you think adding a small amount of medium to the glaze will help adhesion. I'm going to try dip the outside then pour the inside to see if that fixes things (letting them dry between). With regards to firing schedules, I was taught to bisque higher than glaze for earthenware to make sure the impurities and carbon is all burnt out and more won't come out at the lower glaze temp. I wonder if there is still water in the bisque after glazing despite leaving it a day or two to dry. Perhaps I should slow down the initial ramp of the glaze fire; I suppose this could explain why it mostly happens next to the elements because this is where it's getting hot first and is where the water is coming out and dislodging the glaze?
  13. Glaze Problem on slipcast items

    Thanks for your reply. I use the same kiln for bisque and glaze (obviously separate firings). I don't think the bisque is getting too hot, as it's only 15ºc more than the glaze firing. I haven't tested it, perhaps I should get some cones. I'm getting a new kiln in the next couple of months anyway.
  14. Glaze Problem on slipcast items

    I add the sodium silicate in stages over a few days to get the correct consistency, mix with a drill mixer and then let it sit before putting it through a sieve, it's then usually the next day when I use it. I stir it up every time I use it and put it through a larger sieve (a kitchen one) into my pouring jug just in case there are any lumps. I can't imagine there is a layer of sodium on the mould because it does it even on new ones. The surface where the glaze crawls isn't powdery, it's slightly shiny, kind of like a very very very thin layer of transparent glaze.
  15. Glaze Problem on slipcast items

    Thanks for your reply. The clay isn't recycled no, pretty much straight out of the bag, dried, and ground up and mixed with water and sodium silicate to deflocculate, sieved and into a mould. It's by Valentine Clays. I have noticed that about 70% of the problem pots have the problem on the side facing the elements, could there be a problem with some kind of over-firing?
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