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Tim T

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    Nether Wallop, England
  1. John, well done for trying to teach people the basics of this sort of stuff. Unlike many potters I'm from an engineering background and so am familiar with much of these ideas, but you talk to many potters of all levels, from beginners to professionals, and it is totally outside their area of understanding, so either they stick their head in the silica and say it'll never happen to me, or get over excited about whatever the latest scare story is. One of the HSE links (I forget which) is guidance for potters, and is clearly and well written on the risks and on good studio practice for non technical people.
  2. An MSDS data sheet is a good starting point but doesn't really give the full story, it just highlights the risks for high levels of exposure to the substance. There are many other things to consider: For working with glazes in the UK you must either use a lead free or a low solubility lead glaze, where low solubility measures the amount of lead released from mixing the dry glaze in hydrochloric acid. Lead bisilicate releases little lead in this test, so any lead glaze using lead bisilicate can be used. You must ask yourself how much exposure you have to the material. In the UK when looking at long term effects (as is the case with lead) if you are working with it all the time then this is averaged over an 8 hour period (ie a day), or if you work with it intermittently then this is averaged over a 40 hor period (ie a week). So if you are mixing lead glazes for 1 day a week then the exposure can be 5 times higher on that day than if you do it every day. Then you need to look at how the toxic material can get into the body. Lead bisilicate isn't absorbed through the skin, and if ingested through the mouth in the quantities likely in a studio using good practice (i.e. no food or drink, no licking of brushes for applying glaze, washing hands/using gloves afterwards etc) is not going to release a significant amount of lead passing through the system, so the main risk is airborne inhalation. If you look at the HSE airborne limits for lead in the UK, this is 0.15mg/cu.m of lead (corresponding to 0.37mg/cu.m. of lead bisilicate), which is higher than the limits for chromates, nickel, cobalt and vanadium. The question is, how much of the lead in your glaze will get into the air - primarily by using dry materials when mixing the glaze or doing things like scraffito, or by over firing the glaze so it gives off lead fumes in the kiln. And then, how much will be filtered out by your dust mask? This can only be ascertained by measurement, which is beyond the skills or cost effectiveness of studio potters, but guidance from the HSE is in these document: http://www.hse.gov.uk/pubns/priced/l60.pdf http://www.hse.gov.uk/pUbns/priced/eh40.pdf http://www.hse.gov.uk/pubns/priced/l132.pdf Also this is a very useful reference on lead glazes not just in manufacturing, but also on tableware: www.ilmc.org/Publications/ILMCFinalCombo8-02B.pdf Yes, lead presents hazards, but if used intelligently they are perfecty manageable, the same as with many other potter's materials. And it is your choice whether to use it or not. Tim
  3. One tp I've found is not to wash the ash. It is a lot of work, and removes soluble fluxes. Unwashed ash glazed generally have a halo round the edge of the glaze, but I don't mind that, and although a bit more caustic putting on a pair of gloves is a lot less work than washing the ash! Perhaps try it on the inside of a bowl, so it won't run off the pot and upset the others in the studio!
  4. Sutton Taylor's work is wonderful, IMHO. See http://www.oxfordceramics.com/sutton-taylor-1 There are also a few people doing lustreware in the UK following on from William de Morgan's work. If you are not doing pieces for food or drink, lead bisilicate isn't much of an issue as it is a frit that has the lead in an almost insoluble form, though you do need to be a lot more careful if using red or white lead, and lead sesquisilicate isn't as insoluble as the bisilicate. If you look into the chemistry of lead, there is a lot more to it than just the low MP and high refractive index, the chemistry with the silica and the colourants is fundamentally different, so other materials are more alternatives than replacements.
  5. Tim T

    Glazed Mugs Safety

    Oldlady, I'd say that (almost) nobody died from eating their food off lead glazed wares. A well formulated and properly fired lead glaze does not leach lead, which is why it is still used in many production potteries in the world for dinnerware etc without any harm. With studio potters, I think it fair to say that many (most?) don't have the skills to ensure the glaze is safe. As you say, the main risk of lead poisoning was to the person applying the glaze, which in the days of Wedgewood etc use lead ore. Safety legislation was first created in the UK, and eventually the US caught up despite the ceramics industry saying it would be forced out of business due to the expense of not killing it's staff. Yes, many lead free glazes have been developed, but the biggest contrbution was in using lead bisilicate, which is a frit that almost eliminates the absorption of lead into the body.
  6. Tim T

    High Temp Wire Question

    I've played with using plain steel in large pieces to prevent them collapsing in the kiln - about 30" long and from memory 1/4" / 5mm diameter. These are fine for a cone 6 firing if there is no soak, though you'll need something with a higher melting point for cone 10. Shrinkage and expansion are issues - the clay will shrink on drying, and then again on firing, which may be towards or away from your metal armature. Also, of course, the metal will expand when heated and contract again afterwards. I forgot this so had holes in the fired clay at the top, with a few mm of metal sticking out, which had to be cut off and plugged with epoxy. If doing this again I would prbably make the end of the rod cardboard or tape or something so it created space for clay shrinkage/metal expansion, though you may not need this at your scale of work.
  7. Tim T

    Epoxy Thickener?

    I've used epoxy a lot in boatbuilding, and al the fillers used (microballoons, microfibres, colloidal silica) make it opaque - though there may be some specialist ones that don't do this. The simplest approach is to lower the temperature - depending on the temperature round you now, put the epoxy outside overnight or in the fridge before mixing, then mix it up and keep it cool at least for the first part of the set. But do check the data sheet, as a lower temperature will significantly slow down the cure time, and if too low it may effectively stop altogether. To ensure a full cure, once it has gelled and so is no longer going to run it is probably worth raising the temperature to at least room temperature.
  8. Tim T

    Glaze Additive/s

    If the glaze tends to settle out a bit I just add a couple of drops of vinegar, just using bentonite if it is very low in clay and so has severe settling problems. Adding bentonite I don't find a problem if it is mixed in with dry ingredients before water is added. If added to a wet glaze on its own it won't end well. Also, leave the glaze at least overnight for the bentonite to fully interact with the water. I don't believe that adhesion to the pot of the unfired glaze is the same problem, as this involves things like glaze shrinkage on drying. I use gum arabic (with a few drops of Dettol as well), though a friend of mine swears by wallpaper paste.
  9. Tim T

    Element Support Tubes

    Isn't there also an element of keeping the purchase cost low, even if the maintenance costs are higher? Elements using thicker wire would push the purchase cost up, even if the the ongoing maintenance costs from less frequent element replacements were lower.
  10. Tim T

    Stoneware Clay Properties

    How are you defining/measuring plasticity in this instance? Off the top of my head, there are probably at least 3 different factors: - the force needed to move the clay, i.e. its resistance to your hands on the wheel - the plasticity, taken as how much the clay can be deformed without rupturing - how tight a radius can a coil of clay be bent into before cracks appear on the outside - the ability of the clay to support its weight in the thrown object, e.g. if you do a wide shallow bowl, will it collapse The first and last are interrelated, except thixotropy means that a static clay object may support itself, but become more fluid when throwing. Also, obviously, for a given clay changing the amount of water will have a significant effect.
  11. Tim T

    Home Made Kiln Controller

    I've decided that doing my own kiln controller would be an interesting project, but too much of a distraction from making pots. This was exacerbated by my (now ex) kiln controller deciding to overfire to about 1330 (in a kiln rated to 1300!). In the soak, when the elements came on, the temperature readout dropped so they stayed on till it got to the target of 1290, then cut out and the temperature readout immediately jumped up to 1330 and the kiln cooled down to 1285, then the elements came on and the temperature immediately dropped. I guess the elements were causing a voltage drop which then affected the temperature reading. Kiln survived, though the stoneware in it started bloating as it was an oil spot glaze with a long soak, and a while until I noticed. Anyway, I've found that the Bentrup TC507 controller does all I need and more, and a lot quicker to get in place than rolling my own. It can also be used to control a gas kiln if you have the right valves, though I just use electric. So I'm wiring it all in, including a couple of thyristors to replace the relays which should give a longer element life.
  12. Like Curt, I've found digital scales great. The main one I use goes up to 3kg with an accuracy of 0.1g. I also tried one with a lower maximum weight that had a read out to 0.01 g for making small glaze test samples, but I found the actual accuracy was about 0.025g. If you are messy like me, wrap it in a piece of clingfilm to keep it dry when you slosh water or glaze over it!
  13. Tim T

    Bisque Ware History

    For the earliest lead glazes (at least in the UK - not sure about Persia) they just sprinkled some powdered galena (lead sulphide) onto the newly made, still damp pot. Then later, as you say, they made it into more of what we see as a glaze today. Interestingly, in the ceramics factories that John describes, the glazers were paid 6 old pence (about 8 US cents) a week extra because of their shortened life expectancy due to lead poisoning - this affected all but about 5% of them, and of course enough people thought they'd be in the lucky 5% so became glazers for the extra money. There was no safe practice in those days to limit exposure through dust etc, so it was inhaled and ingested by the workers. From memory, about 400 died a year with lead poisoning on their death certificates (and probably lots more who also suffered from lead poisoning but had other causes on their certificates. It was this that caused a drive to make safer glazes in the UK, making the glaze much safer to apply by using the (almost) insoluble lead bisilicate frit, lower lead content in glazes etc and this brought deaths down to zero. When moves were made to introduce similar regulations in the US, all the factories were up in arms about it, saying that it would make them to expensive, destroy the business etc etc, which of course it didn't. It was this drive to protect otters with very high levels of exposure to lead in the workplace that started the reaction against lead glazes, not the very very much lower exposure of users of pottery to small amounts of lead leaching out of glazes and, as I'm sure you all know, lead glazes are still used extensively in dinnerware in commercial production by factories (at least in Europe).
  14. Tim T

    Stoneware Limit Study

    Hi Nerd, Just picking up on this thread again. I've done a bit of work looking at clays for water content and plasticity, using the Atterberg test for moisture content for upper and lower plastic limits. Also, in the absence of a ny proper lab kit for this, a drop test to measure the actual plasticity - make up a 50mm ball, drop from a height of 2m into a bowl, and measure the diameter, plus take a piece and measure the MC - not perfect but indicative. What I initially assumed was that a wide moisture range would correspond to a high level of plasticity, but as I test more clays I find this isn't always the case. At the same time I put it through a set of sieves, which gives a breakdown on the medium to larger particle sizes. Just need to find time to see if there is any link between particle size distribution and plasticity. Also, with hindsight, I should have done the drop test at say 3 different MCs.
  15. Tim T

    Quartz Inversion

    Note that in the UK (and possibly the rest of Europe), quartz/silica and flint are not exactly the same. Quartz or silica refer to the same material as in the US, generally made from sand. Flint is, as its name suggests, made from stones of flint. They are calcined (to about 400C I believe) to make them weaker and then ground down. Generally flint melts a little bit earlier than purte silica, and also has a bit of calcium in it as the stones pick this up from the chalk soils they are generally found in. As a matter of interest, but total irrelevance to here, the farmer across the road from us harvests flint, getting it out of fields both to sell and to improve the soil. He's just got 1000 tons out from another farm. This is mostly used for flint finishing to house walls, I suspect the share that goes to potters is pretty low.

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