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Everything posted by AndieF

  1. Hi Rae, thank you. Yes, I'm aware of that, if you read my very first post at the top of the page, I discuss that very point, the rate of rise of the temperature of my kiln is pretty much in the middle of the two rates orton give on their chart in my possession, so I'm interpolating between those values to arrive at a target temperature. The temps you gave are for cone 5.5 on the Orton chart I have.
  2. Thank you Rae Reich. The temperatures you gave are for cone 5.5 not cone 6. I am using large cones, not kiln sitter cones. I bought the cones recently from a German supplier, so unfortunately I'm not able to say how they may have been handled in the past. I wrote to Orton last week but haven't received a reply from them. I believe you are most likely correct that the cones are not good anymore because the glaze results just don't match up with what the cones are saying. A cone 6 glaze was not properly melted when the cone showed it to be a hot cone 6 firing. The cones seem to be difficult to obtain here in Germany, but I've seen that another supplier has rings on sale which shrink after firing in a way proportional to the heat work imposed on them. Accurate measuring of the shrinkage of the ring and an accompanying chart enable the temperature reached to be determined.
  3. One of the pots that came out of that last firing described above (controller set to 1210°C, cone 6 completely bent over and cone 7 half bent) looked under-fired. There are lots of pits and holes in the glaze where it looks like it hasn't melted properly, especially in the rim, also the colour doesn't look anything like it's supposed to. This is a glaze rated for between 1200°C -1260°C. I refired the pot at a higher temperature similar to my first firing, controller set to 1240°C, all cones (5,6,7) went completely down - according to the cones it looks like at least a cone 8 or 9 firing again. Now the glaze looks like it's supposed to, the colour looks like the manufacturer's pic and the glaze surface looks smooth. Haven't heard back from Orton yet, but I really don't trust these cones. I notice now that the supplier of the cones has written on their website that in their tests the cones fall at a temp 20 -35°C lower than stated in the charts supplied by Orton. Most curious, but that seems to fit with my experience! I'd like to get some more cones from a different supplier but haven't found anyone yet.
  4. Thanks Sputty, I'm very glad for your input, it seems the cones might not be good then. I've just written an email to Orton!
  5. Thank you Neil, I fired the kiln again yesterday with a reduced target temperature of 1210°C, however the cones indicate a hot cone 6, as cone 7 is already starting to bend. Both cones show the same bubbled texture as my first firing. I'm not sure if the bubbling could be due to insufficient venting because the 04 cone I used for the bisque firing looked perfectly clean without any sign of bubbling, if venting was an issue with the cones, wouldn't the 04 cone show the same signs of bubbling? Maybe the cones are not to be trusted. Min, I'll try leaving the lid propped until 750°C on my next firing just to be sure the kiln is well vented. Ultimately I'd like to drill a hole to be able to see the cones, although the prospect of doing that scares me a little bit! Thanks again!
  6. Thank you Min for your advice. I'll look into getting a drill suitable for stainless steel - it's quite a hard material? This is the company that makes the kiln, http://www.marienfeld-dortmund.de/brennöfen.htm (Sorry, it's all in German) looking at the pics on their website there don't appear to be any spy-holes installed. What would be the consequences of the kiln not being sufficiently vented? The thermocouple installed is a platinum-rhodium/platinum type S. They are supposed to be more accurate and suffer less drift than the Type K often installed by other manufacturers.
  7. Thank you Babs and yappystudent for the advice, I have several welding masks I could use to look in the kiln. Babs, I spoke to the manufacturer of the kiln, and they say that the bricks are 'breathable' and there is no need to vent.
  8. Thank you Pres. Maybe I really should drill a hole so that I could observe the colour of the pots and the cones? Would it not damage the kiln if I drill through the stainless steel cladding? How big a hole would I need? Drilling the brick should be easy as they are very soft.
  9. Thank you Min. The clay is rated from 1200°C - 1260°C, as is the glaze I used. At the rate of temperature rise I had, I calculate that a cone 9 would be about 1268°C, so slightly above the ratings for the clay and glaze. There's absolutely no blistering of the clay and the colour of the Glaze actually looks better then the manufacturer's photo with lots of small variations of colour. Would a lower temperature produce a more muted effect? If there is an offset, wouldn't that be linear over the whole temperature range? In which case the glass thermometer might work?
  10. Thank you Babs. The controller is quite primitive, so it's not possible to enter a temp rise. The temp rate of 104°C/hr was calculated from manually noting the temp every ten minutes leading up to max temp. The kiln doesn't have any peepholes either, perhaps I should drill a hole!
  11. Hello all, I'm new to the forum and to making and firing pottery! Yesterday I fired my kiln (MKO40 40 litres, electric, it's a German kiln) for the second time. The first firing was to bisque a pot and it went well, I was aiming for cone 04 and it bent almost perfectly. Yesterday, I did my first glaze firing. I was aiming for cone 6 and set the temperature to 1240°C with a 6 minute hold. I monitored the temperature regularly for the last 100°C leading up to 1240°C and it seems the rate of rise in temperature was 104°C/hr. Looking at Orton's chart I see that for a rise rate of 60°C/Hr a Cone 6 is 1220°C, and for a rate rise of 150°C/hr a cone 6 is 1241°C. As my rate of rise is pretty much in the middle of those two, I see I should perhaps have set the temperature to 1230°C. When I opened the kiln this morning I was surprised to see that all the cones in my pack (5,6,7 from left to right. I clumsily snapped off the cone 5's tip) were completely down with a bubbly surface. I'm not sure what temperature was reached but it must have been at least cone 8 which at the rate of rise of my kiln is, according to Orton's chart, about 1258°C. So my kiln is overfiring by at least 20°C. Should I fire my next pots with the temperature set to my target temperature minus 20°C? I have a glass thermometer that goes up to 400°C, is it worth heating the kiln up to say around 350°C with the glass thermometer positioned very close to the thermocouple(type s) and see if there is an offset? Would that be a reliable test? The glaze actually turned out really nice, but I am rather concerned that the kiln is a lot hotter than I thought! Thank you Andie
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