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curt

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  1. When I broke some test bars recently to look at a fired clay body, I chanced upon a lovely little bloat! We were talking about bloating recently so I thought I would share these pics courtesy of my hand-held zoom cam. This is a black brick clay body sporting 4.3% molar Iron and 1.3% molar Manganese. I don't think it was ever designed to be fired to Cone 10, so of course that was the first thing I did. Well behaved for the most part, fine for sculptural or decorative work, although I don't think you would be choosing it for your new line of functional dinnerware . This sample was fired in a gas kiln with medium reduction. The bloat you see here occurred very near the surface of the test bar. It is about 0.5 of a millimetre ( 1/50th of an inch) in diameter so pretty small really. I would not have seen it on the surface of this otherwise rough-ish clay body if the break in the test bar had not split the bloat close to right down the middle. Note the smooth surfaces and spherical shape of the bloat bubble inside. The inside edge is strikingly curved and clean. The "black glass" also appears to be evident inside and around the edges of the bloat. In fact the whole inside looks like it could have been coated in a shiny iron. The final picture just for overkill is a reverse lighting image of the inside of the bloat where the dark patches are actually the reflection of the microscope light...
  2. Finally located the sample of the brick clay I was referring to a few posts above when we were trying to identify the clay type in this thread. I had to (gulp!) break my sample bars to see what the interior clay body looked like. The top two pieces are the bar fired in the wood kiln, the middle two pieces are the bar fired in electric kiln, and the bottom two pieces are the bar fired in the gas kiln. All Cone 10. Note the big white specs, which I am pretty sure are flint or similar. They may look solid white to the naked eye, but close up (see pictures below at 10x and 700x magnification) it is clear (to me at least) that they are some kind of quartz-like silica material.
  3. curt

    COE and Thickness

    Magic Magnesium to the rescue...the anti-crazer of choice! ...hmmmm, or was it the chunky boron component that fixed the crazing?.... (scratches head). (Always love how Insight ignores Boron when calculating RO Unity... - wish I could change that) Or was it the combination of magnesium AND boron??? (starting to feel light-headed at this point). Will take a long bath and think about this. In any case, I like the short soak just off the top, Min. Such things are much prescribed but generally little reported. Thank you! EDIT: I think the milkiness is due (aside from simple thickness) to the boron?
  4. curt

    Temperature Algorithm?

    I just saw something like what I have suggested over at reddit. A bot which converts imperial measurements to metric in posts. Made by user u/Bot_Metric I am investigating and will pass on what I learn.
  5. Yep that is crazing. A very normal problem. Your glaze is shrinking more than your pot does as it cools, that is what is making the cracks. A double layer of glaze may be providing too much flux, so I would try getting a thinner glaze layer first and that MIGHT fix the problem. If that doesn’t do it, then get a new glaze, or reformulate the one you have to contain less fluxing power. Not much you can do with the claybody I think, as earthenware is earthenware...
  6. The hairline cracks are almost certainly what is called crazing. Do a search of these forums and you will find it is widely discussed. Can you post a photo? the leaky cup may be due to various causes. As Babs is saying we need to know more detail of your clay body and firing to make suggestions.
  7. Another approach instead of adding kaolin is to remove some of the fluxes you have in your glaze. Both the crazing and the runniness evident in your picture strongly suggest (to me anyway) that your glaze has way too much flux in it. I might look at reducing or removing the gerstley borate as a start. You will probably lose some of that baby-blue borate flush evident at the bottom of your test piece, bu since borate is such a low temperature melter it will probably go a long way to reducing the runniess. just a thought.
  8. Looking again at this pot, isnt’t it the bottom part of a lidded pot? Has the lid been lost? We can see that the outside and MOST of the inside of this pot is glazed, but not the gallery (the uppermost part of the inside) - that is just the raw clay body. Lidded pots are often (always?) made this way (gallery unglazed) so that they can be fired with the lid on. Otherwise the glaze would melt and stick the lid in place during the firing and you would never get it off! Probably this is how this pot was fired. Importantly, the light clay we are seeing is where the gallery is, ie on the inside where the lid would have been sitting during the glaze firing. Since this pot was originally almost certainly fired in reduction (a gas or wood kin rather than an electric kiln), the presence of the lid would have prevented the kiln gasses from touching the gallery area where the kiln was sitting, preventing the clay where the lid was from getting reduced (blackened). So that unglazed clay directly adjacent to where the lid would have been during firing has stayed unreduced (lighter brown), much as it would have been if the whole pot would have been fired in an electric kiln (oxidation). If you look closely there are still white specks even in the light brown part of the clay, since they are part of the clay body material the whole pot was made from. Just a theory, but it seems to fit the evidence. Also, what kind of clay? I think glazenerd may be right that it could be a brick clay of some kind. In fact, where I live we have a brick clay that looks just like this - right down to the white specks - when fired in reduction. Significantly, it has a couple % Manganese dioxide in it as the main (darkening) colorant. Will try to post photos if I can locate a sample. Who knows, maybe the clay even came from here! Although that would be a long journey for some raw clay!
  9. White specks are probably chunks of flint.
  10. curt

    Curious about porcelain

    The cream is almost certainly due to a small amount (fractions of a percent) of iron in the clay. I assume you are firing in an electric kiln when you see that cream color. In a gas kiln the cream would instead be a blue with a bit of reduction. As for the pink, not sure... Others may know. pictures?
  11. curt

    Will Re-Bisqueing Stop Bloating?

    Hi Plum, I can't find any chemistry for Standard 266, but I can see plenty of comments all over these forums and the internet at large over the last 5 years or so that indicate that 266 is a known bloating suspect. Probably because of its readily visible high iron content. However, the reality is that you probably don't know what is causing the bloating problem (and neither do I!). If the cause of the bloating is due to insufficient organic burnout in the clay body (and it still could be this even with high metal content), then the firing changes need to be made to your bisque firing schedule, not your glaze firing schedule. Perhaps reread the second half of my post above, where the paragraph starts "Finally, regarding the bisque firing". I would try this first, since it is the "easy" fix as it just means tweaking your bisque firing schedule, which should have little or no impact on any other clay bodies or glazes you are using. You would want it to be this kind of bloating problem, because there is actually something you can do about it. If the bloating is due to metal and/or carbon concentrations rather than organic matter, then I still would make the changes to your bisque firing programme anyway, since enabling adequate decomposition and offgassing of sulphur may still help avoid what I have just seen Laguna refers to as the "black glass" problem (just another way of describing bloating due to internal reduction in the clay body). Finally, yes, you can try firing lower than cone 6 in the glaze firing, but as you appreciate this may create significant difficulties with other clays and glazes you are using. I do not fire to cone 6 myself, nor do I use 266, so others are probably better placed to advise on this. Good luck, hope you get it fixed!
  12. Not sure if this is the right place for this, but I have a suggestion for how we might save time and headaches when dealing with temperatures in threads. Not sure if it can be done, but the folks back in the lab will probably know or could cook something up. Since I work in Celsius, whenever I come across a forum post quoting temperatures in Fahrenheit, I have to stop and make a conversion, either by consulting some lookup table, or using a conversion app of some kind, or (GF!) using a calculator (too hard at that point). After the 432nd F to C conversion this month, it occurred to me that it would probably be possible for the software guru’s to add a small bit of code to these forums so that when I am drafting a post and include a temp in Celsius, as long as it was in a very specific format (eg “1280 C”), the forum software would automatically recognise that and automatically calculate and insert the equivalent temp in Fahrenheit behind my original in Celsius, using a little algorithm or something similar. Without me or any reader having to do anything or make any calculations ourselves. And of course vice versa for Fahrenheit to Celsius. Hope it is clear what I am getting at? Is that possible does anyone know? Has it been implemented on any other websites or forums you are aware of? Or is it already a feature on this website that I just don’t know about? Thoughts or comment appreciated.
  13. curt

    Will Re-Bisqueing Stop Bloating?

    Callie and timbo pretty much have it, but I wanted to add a few more thoughts. As identified, the two main causes of bloating are 1) insufficient burnout of organic materials in the clay, (I will come back to this below), and 2) metals in the clay which will not burn out at any temperature. These metals are mostly harmless at standard bisque temps, but at the higher temps experienced during the glaze firing these metals can melt and form gaseous pockets in the clay. From a clay body preparation standpoint, we would prefer these metal particles to be fairly fine grained, and to be mixed fairly homogenously with other clay body materials, so that in the overall vitrification they don't congregate in little locales. Larger grains of metal, or poorly mixed clay bodies which leave areas of dense metal population, are the seeds of bloating trouble. So yes, slurried, filtered clay bodies produced by reputable manufacturers with good machinery should be best in this regard. It also helps to have a clay body which is more "open" meaning some large particle size material which promotes gaps for gases to exit. Heavy iron bodies, or bodies with "junk" in them due to poor preparation, or overly "tight" bodies (or all three of these together!) tend to be bloat-prone. During the glaze firing, ideally we would like to prevent/avoid these metals from melting until relatively late in the glaze firing process, so that they do not have time to form pockets of gas in the clay that turn into bloats. If these metals melt too early, before the rest of the clay body really starts to flux and vitrify, then the metals run around and hook up in a small area and form a pocket which get sealed. If other clay body materials which need to offgas (say, whiting) get trapped in this little pocket as well, then they can add to the bloat. As others have said, bloating can be aggravated by excessive heatwork (ie, firing longer than necessary to achieve vitrification of the clay body, too long or too slowly at higher temperatures, or excessively long holds at the top temperature). Bloating can be further intensified by a reduction atmosphere (in a gas or wood kiln) which causes some metals - particularly iron - to flux ("melt") much earlier than usual, and to become more active than usual for the remainder of the firing. Another related cause/promoter of bloating may be a particular chemical reaction between iron and sulphur, which often arrive together in a clay body which has been mined where there is lignite or coal present (eg, many fireclays). Sometimes referred to as "black-coring". I think glazenerd had some research on this in another post a while back? This particular chemical reaction may cause "hyper-reduction" to occur. Finally, regarding the bisque firing. (Plumcreative note:) The bisque is where the bloating is fixed, not the glaze firing. The bisque firing is where the organic burnout (cause #1 above) is accomplished. As timbo and Callie said, if you have organic matter in your clay (such as paper, small tree hair roots, animal hair, etc...), this needs to be burned out in the bisque at around 800 Celsius. At that temperature chemically bound water has already been released, and there is enough heat that organic matter in the clay body can ignite and decompose IF sufficient oxygen is being supplied. Note that just passing by this temperature on the way up WILL NOT DO IT. Program a one-hour hold in to your bisque firing at 800C. Be mindful that the burnout needs to get ALL THE WAY in to the center of your clay body, so if your pots are thicker it will take longer - so program the hold for TWO hours. Leave all the bungs and peeps open on your kiln until at least 900 Celsius, and yes by all means have the vent on if you have one. If there is not enough oxygen in the kiln then burnout cannot occur, and worse you may start to get localized reduction of metals in the clay (yes, even in an electric kiln) if not enough oxygen is available, and that is what you DO NOT want, as sealed pockets can form in the clay body even at that bisque stage. You will not know those pockets are there because at bisque temperatures the clay body does not vitrify or become flexible enough for them to manifest, but trust me there are there, and they will expand and show up during the glaze firing later when the clay body gets soft. This is also why you do not want to stack pots which are bloat-prone - because it prevents oxygen getting in. Anyway you want the bungs wide open during the bisque to let water steaming of the clay to occur so you don't wreck your kiln from the inside out with moisture. DO NOT overfire your pots in the bisque. Don't fire them above recommended temps, and don't fire them twice or more. As per the above, this will do nothing other than make your bisqueware too "tight" (less absorbant of glaze slop - er, unless you want that), and may well promote bloating by exposing clay body constituents to excessive heatwork. As an aside, while I mostly agree with timbo above, I think he is confusing sintering with vitrification. The sintering that occurs during a bisque firing does not seal up the clay body (unwanted metal pockets aside) - it only welds the very tips of clay particles together, which is what makes a bisque pot stiff and resonant, but definitely not sealed up. You know this because bisque ware is incredibly absorbant when dipped in glaze slop. It is too late to fix your bloating problems in the glaze firing (but you can make it worse as per the above). Even if you were to try to replicate bisque-like firing conditions during the early part of your glaze firing, the problems is that many of the materials we use in our glazes melt early (eg Boron is fully melted by 700 C) in the glaze firing, in effect sealing up the clay body underneath, further discouraging any more off-gassing and hence promoting bloating. In summary, bloating has to be fixed in the bisque firing if it is due to insufficient organic burnout. If bloating is being caused by metals in the clay, make sure your kiln is well vented (un-bunged) and avoid early, unintended reduction, and don't overfire and you may still avoid it. Users of paper clay, reclaim clay, or native clay (which often has lots of iron), take note. These are likely to be issues for you.
  14. curt

    How to open Venco wheel

    No need for these drastic measures. I have used these wheels since forever. Yes they are always easily and quickly serviced from underneath, no need to rip them apart. Parts are regularly available, and they last for decades. The Volkswagen of pottery wheels we say out here in Perth (where these wheels are actually manufactured). Not the most luxurious but very sturdy. Brendan if you Message me privately I can send you the contact info for the guy everyone uses to service these wheels here in Perth, or tell you where to get parts if you want to have a go on your own. on the fishing, hard to say....they catch some pretty big fish up in Queensland....but our sunsets are surely better?....
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