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Everything posted by Min

  1. So there is no hood style vent above your kiln or a downdraft vent from the bottom or side of the kiln? If this is the case then generally speaking you would leave the side peep hole plugs out until the kiln reaches red heat then put the bottom one in. The top one on the side could be left out for the entire firing or put in an hour or so after the bottom one. If you use a dark claybody then I'ld leave them both out for the entire bisque firing. There isn't really a hard and fast rule about when to put the plugs in, some people will leave the top one out for the entire firings. Could you post a picture of the vents in the lid for clarity? Welcome to the forum.
  2. For completeness of your testing you might want to do a stress test to check the fit of the glaze. The one you like the best with less clay and using nep sy is going to have a higher coefficient of expansion than the original so it might craze over time. Also, I'ld try to use a lower sg and dip if you plan on using this glaze a lot, much faster to dip than spray and for a clear you aren't looking for layering spray glaze effects. Link here if you need it on how to stress test for crazing. Thanks for posting your results.
  3. Could very well be a glaze thickness issue. A quick look at the glaze from the Amaco site shows the difference thickness can make. Before glazing more pots I'ld take a scrap/test pot and do one dip all over then dip a second time for the top half of the pot and see if you get the rutile/turquoise colour is better. I'ld also measure the specific gravity of the glaze so you can replicate results. (lots of posts about measuring specific gravity if need it, do a search on the main page) Welcome to the forum.
  4. Other thing I would look at is how thin the clay is in the area I pointed to with the arrow below. If this is too thin the rim will slump. I'ld try break a couple slumped ones open and look at the profile.
  5. I'll forward your post to admin to resolve.
  6. @Mark C., yup! That makes 3 of us now that are saying the same thing.
  7. Speaking of rude surprises, Ferro Frit 3249 full bag price locally is 432. If I could drive across the border and pick it up in Seattle it's 225. Even with the currency exchange rate that's just ridiculous. In general I have found that there has been a steady increase in price over the years. If there is a dip in a chemical price it takes a while for my supplier to reflect that but if there is an increase (like for spodumene because of the lithium price going up for batteries) they are quicker to adjust prices. I have found that if a bigger ceramic supply place like Seattle Pottery Supply carries a somewhat uncommon material and stockpiles it that their prices will reflect the old price for longer than a smaller ceramic supply place does. Example of this was when I went to buy a full bag of grolleg, price went from 60 to 110. Our gas prices are expensive here, filled up last week at 1.70 CA a litre (about 6.50 a US gallon / 7.70 a Canadian gallon), I think that drives up the cost of our clay especially since it's all brought into the area from either the US or Alberta.
  8. First off I have to say good on you @Hannah 112 for reading up on the glazes you are using and questioning the term "food safe". Cadmium and lead release from ceramic glazes are regulated in the U.S. This doesn't mean the glazes can't contain either of those oxides, it means when leach tested the glaze mustn't leach more than federally acceptable levels. Much of inexpensive imported crockery is made with lead glazes but they are stringently tested to comply with the regulations. Lead release info here and cadmium here. Lead is still available to purchase in the U.S. for studio ceramic glazes but it's very uncommon to use it. Ceramic stains containing cadmium that has been processed into an inclusion stain are common. They are used in mostly in underglazes and glazes. The processing of the cadmium into an inclusion stain renders the cadmium to be locked into a form that doesn't leach. (or if it does that it falls below federally acceptable levels when used in underglazes and glazes) Barium isn't included in materials tested for in the regulations therefore there are no standards that glaze manufacturers have to comply with. If a glazes isn't well formulated and fired to maturity (this part is really important) then levels of barium (or any oxide) leaching are increased. I personally would never use a ^6 glaze containing barium on any surface that has the possibility of coming in contact with food. There are differing views on this, in England for example ^10 barium glazes are very common. There is a lovely glaze barium blue colour that is elusive to achieve without the use of barium but substituting strontium (at 75% of the barium amount) gets a somewhat close colour. There have been many discussions on barium here on the forum, which can be found if you do a search on the main page. If you do decide to use a ^6 glaze that contains barium on a surface that could come in contact with food I would suggest getting it leach tested. Brandywine Science Center cost was around $30- for one oxide last time I checked. Welcome to the forum.
  9. The VC 71 has been around for years and I've read many accounts of it being problematic. Spar is high for a cone 6 glaze, boron is low and it will fire to a gloss if taken higher, these all point to it being matte because it's an underfired higher fire glaze, not a properly formulated matte glaze for cone 6. It's had a few alterations along with name changes over the years. With approx 3 copper carb + 5 rutile it became known as Xavier Warm Jade. Ron Roy altered the Xavier's Warm Jade recipe to be more durable when fired to cone 6 by decreasing the spar, increasing the boron and giving a slight bump to the silica. Much better glaze chemistry wise, it does fire to a gloss. Comparison of the two glazes below. Obviously it's your choice to use it as is but if adding zircopax to it makes it even more underfired then reduce the silica content of the glaze by about 1/2 the zircopax content that you add. Between 8 - 12% zircopax should give you a solid white.
  10. I have a friend who used to pick up free clay from a community studio. She would find ribs and all sorts of crap in it. It wasn't worth the hassle of getting free clay when she had to pick through it all to make sure nothing in it would do a number on her pugger and/or contaminate the clay. There is also the time it takes to clean out the pugger between batches, takes me about 3 hours to clean out my Bailey and put it back together again.
  11. Idea from Deb Schwartzkopf from Rat City Studios, where they combine cone 6 clay and scrap glaze and form it into bricks that they fire to cone 1. https://ratcitystudios.com/blog/2016/5/2/puot810gtp1t6nqtgxjl7iw1qh28ss Insofar as hazardous waste materials the four main materials you need to pay special attention to are lead, cadmium, chrome and barium. EPA has regulations on how those need to be disposed of. Biggest risk to the potter in the studio is from silica dust.
  12. A lot of bodies can go through the inversion zones with no issues, even with a refire. A lot depends on the body, the evenness of the temperature though both the kiln and the pot itself plus the glaze fit. Perfect storm would be a heavy application of a high expansion glaze on one side with a low expansion glaze on the other and a rapid increase in temperature of the kiln where parts of the pot heat and cool faster than others. Yes, for refires (which is essentially what you are doing if you fire the clay to maturity in the first firing) go slow both on the rise up and down. If I have to refire a glazed pot, like the one of my avatar where the pot is different thicknesses across the base, if I don't slow the firing down for the quartz inversion I get cracking every single time. (If there is cristobalite in the fired clay then the firing should also be slowed down in the 220C range but this isn't as common) Since the temperature gradient isn't going to be even I would suggest going at 40C from around 530C up to 600C. If your kiln cools quickly then slow it down to this on the way back down too. Again, all this might not be necessary but if you get dunting cracks then give it a go.
  13. Yes. Second firing (glaze firing) could even be a lowfire glaze. Brushing glazes might also be an option as the gums they contain help bind the glaze to the pot. To glaze a vitrified piece is easier said than done though. edit: other consideration with this method would be the firing speed of the glaze firing. You might need to slow it down through the inversion zones to avoid cracking issues.
  14. If you look at the siphon feeding Critter or EZE sprayer or the Bailey rebranded version of the Critter they use approx 2 C.F.M. @ 30-90 P.S.I. or in metric approx 56 litres per minute. Problem with too small a compressor is you will be standing around waiting for it to recharge longer than you will be spraying a pot for. I don't get the powdery look on vertical surfaces unless I have the gun too far away from the pot. The drier the glaze is as it hits the pot the more powdery it will be. There is a balance between having a low enough specific gravity to create a wet puddle of glaze that you "paint" on with the gun and yet not so low as the glaze takes too long to dry or excessively wets the pot causing runs or blistering. You can add gum to toughen up the surface but it will slow down the drying time but I've never found it necessary when the glaze contains enough clay. edit: As well as a respirator a comfortable pair of hearing protection ear muffs is a good idea while spraying.
  15. What properties do you need in the final piece? If a high porosity is okay then a lowfire body might be fine but it won't be very strong, if not then I'ld look at the porcelain bodies. Using the term porcelain is a bit misleading since many "porcelain" slip recipes also contain coarse particle fast casting ball clays. If you are just starting out with slip casting I would recommend the book "The Essential Guide to Mold Making & Slip Casting" by Andrew Martin Welcome to the forum.
  16. Since vitrification and not just maturity is required I'ld start by measuring the actual porosity of your fired stoneware if you haven't already done that. Could run some clay bars through at the same time to see how much it slumps when fired to maturity. Link here if you need it on how to do those tests. Welcome to the forum.
  17. I have sprayed a lot of glazes, it does take some time to learn how much glaze is needed for good coverage. It's common to use too little glaze rather than too much. When I first started spraying glazes I did a kiln load or two using the method outlined in this really good article by Roger Graham. By using Graham's method of measuring out the volume of glaze and doing an estimate of the surface area I found it got me in the ballpark of how much glaze I actually needed. Once I got this figured out I could then do it by eye which really speeded up the process. One thing I disagree with from Graham's article is his feeling that gravity feed spray guns are far better than siphon (bottom feeder) spray guns. My main spray guns were Critters, very few parts to clog and replacement parts available. I swapped out the glass jar for a plastic pint jar and mounted the sprayer on the lid of that. I did use gravity feed ones also, if you go this route I suggest throwing away the little screen that is found inside them. In either case a well sieved glaze is crucial. I sprayed most glazes at around 40-45psi. Re the powdery look, some people go for a thick velvet look to the glaze to know when they have built it up thick enough, most glazes are okay like this if the glazes are well fluxed and they will smooth out during firing. If you use a stiff glaze this doesn't always happen and you can have a pebbly finish. For vertical surfaces I prefer to move the gun in closer and follow a small puddle of glaze around and up and down the pot. I would do three times round the pot total. For flat surfaces such as platters it's hard to avoid some of the powdery look but by increasing the water content of the glaze and moving the gun closer to the pot it can help. Further away the gun is the more time there is for the glaze to dry before reaching the surface of the pot.
  18. In Canada Tuckers supplies the Spanish Sio-2 Black Ice and also Black Stoneware. It is expensive clay but ordering from within Canada has it's advantages. Since you are in the lower mainland of BC you could ask if Greenbarn could bring it in for you the next time they order from Tuckers. edit: Tacoma Clay Art Center carries a couple black clays. I have had Greenbarn bring in claybodies from there also. When the border opens back up again it's not that far a drive from the lower mainland to pick up clay from either.
  19. GA6-C is very similar to an old glaze called Gnu Blue. It was notorious for being a bit tricky getting the thickness right to get the blue from the rutile. If you do try the GA6-C (such a catchy name) I'ld add a bit of cobalt to it to encourage the blue side of it. The ceramic supply place here has samples of Ravenscrag Floating Blue, it always looked a bit muddy to me. I think if I was trying it I'ld swap out the rutile for titanium dioxide instead and use a bit less of it, for every 1 of rutile I'ld try 0.9 of titanium dioxide instead. Both those glazes rely on the rutile for the colouring and or varigation and rutile is one of those materials that can be vastly different between batches and make or break a glaze. I haven't seen the Ravenscrag or Alberta celedon. Weigh the roasted Alberta Slip or Ravenscrag after roasting it when the recipe calls for roasted material. Just like when a recipe calls for calcined kaolin, you weigh it after being calcined.
  20. At cone 6 you need either boron or zinc to get a good melt. I don't know the price of zinc where you are but I'm going to go on a hunch that doing a Bristol glaze (fluxed with 10% + of zinc) is going to be more expensive than using 20% +/- of one of those frits. Tin wouldn't be an absolute must for any glaze with either Ravenscrag or Alberta Slip, there are some recipes from Digitalfire and Ravenscrag that do use it though. Which recipes are you looking at? I "roast" rather than do a complete calcine of them to avoid them starting to sinter and get gritty. I go to 1000F and soak for an hour using a shallow bisque bowl. Just program the schedule in yourself with a top temperature of 1000F then hold for 1 or 2 hours. You can go fast up to 1000F, along the lines of 400F/hr/1000F/60 to 120 hold
  21. Main issue with this idea is the raku glaze is a lowfire glaze and you will be firing it up to cone 6. The lowfire raku glaze is going too in all likelihood way overfire at cone 6. I know there are some lowfire glazes that are okay being fired hotter but the majority aren't, they blister and / or run etc. If you are really keen to try it I'ld try a test tile with the lowfire raku glaze on it, doesn't need to be raku fired, and put it in a little saucer of clay and fire it up to cone 6 and see what it does. If that test comes out okay then I'ld consider a very light application of your cone 6 glaze on the raku pot but would fire it on a waster slab to catch possible (probable) glaze runs. This is assuming your claybody can go to cone 6, most commercial raku bodies can. Welcome to the forum.
  22. Admin has looked into both trying to remove the spaceship icon and also getting the post and positive reactions counts back to specific numbers when over 1K are reached but unfortunately at this point in time those are not things that can be changed.
  23. Since you are taking the time to mix up the test batch I'ld try and get a couple more results from it. I'ld mix up 200 grams of it with around 16% kaolin and dip a test tile. Add the rest of the kaolin to bring it up to the 19.5% and dip another test tile. Then add the 2.5% zinc and dip a third test tile. I'ld also place the test tiles on waster slabs since the glaze could run more. (which would probably distort the cobalt brushwork too) Try one dip over all the test tile and a double dip on a top corner.
  24. Wouldn't be my first choice to counter yellow tinge but wouldn't hurt to try it. Don't forget that bone ash acts as both an opacifier and adds opalescence to a glaze but in such a minute amount it likely won't. Could also try a very tiny bit of blue stain, in the order of 0.01% I would be looking at a different kaolin first. edit: on the molecular level there isn't a huge difference between Marks clear and this one, mostly the zinc, silica and alumina. Easy way to get rid of some of the bubbles would be to drop the zinc and increase the spar.
  25. Do you have an analysis for the kaolin you have? It can be quite interesting to fire material samples up to glaze temps to see their melt and colour. Just a 1/4 teaspoon or so in a depression in a slab of clay. Simplest thing would be to try it with a cleaner kaolin. Can you get grolleg? If that still doesn't make it clean enough see if you can get a small amount of mahavir feldspar and swap the custer for that and add 2 more silica to balance it out. If you can get away without using bentonite with your grolleg do that, if not use veegum or bentone ma (aka macaloid) to suspend the glaze. Third option would be to use nepheline syenite and bump the silica up to 33.
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