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tinbucket

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Everything posted by tinbucket

  1. One thing to note about colored slips - the more vitrified the slip is the more vibrant the color will be compare to a less vitreous slip with an equal percentage of stain. But the more vitreous your slip is the more problems you may encounter when matching shrinkage etc to your clay body. Green and blue are easy/cheap, I have used up to 20% stain to color a clay body. I would suggest doing a test of 5, 10, 15, and 20% stain additions to weight of dry material then narrowing it down from there. Red encapsulated stain is much more expensive and it might be wiser to use red underglaze instead of red slip or use the red slip sparingly. Below are some tests that some fine people have done adding stains to clay. Keep in mind their base is most likely very white and vitreous, which maximizes the potential of the stain addition. https://www.jakecorboy.com/colored-porcelain https://glazy.org/u/derekau?base_type=400&analysisName=umfAnalysis
  2. I have never used sodium silicate but when I use Darvan I have used 0.2% And 40% water, both of the weight of dry ingredients. That means I multiply the dry ingredients times .002 to get the amount of Darvan and .4 to get the amount of water. So for 10,000g of dry material I use 4000g water and 20g Darvan (add Darvan to water, mix well, then add dry material). This is just starting point and the day after I mix the slip I measure the specific gravity and observe the viscosity of the slip to see if it needs adjustment. It sounds like you slip could possibly be over-deflocculated. Another thing to consider is the way gerstley borate gels, but I'm not sure 2% could have that much impact.
  3. I agree with you that making a mold and slip casting would give you the most desirable result. Any other method will fall short of slip casting when it comes to uniformity and thinness imho. One thing to consider is the heat that can build up in an enclosed light fixture. If you use an incandescent bulb and don't have any venting it will get hot. Also since the form will be round on all sides and translucent porcelain is usually somewhat pyroplastic, you might consider making a cradle of sorts to support the piece in the firing (similar to th way bone china is fired). Best of luck. I hope you post pictures of the process and results!
  4. If you notice both examples of running/bleeding you posted are blue, most likely a cobalt (carbonate or oxide) wash rather than an underglaze. If you are after blue, I would use a cobalt wash rather than underglaze. Unless it is very thinly applied, cobalt will have a tendency to run or bleed.
  5. Brent is a relatively small American company. Their customer service is easy to reach and very helpful, they might be able to advise you on the repair
  6. I have experience with it, mostly bodies I mixed myself. The higher you go, the lower the absorption, the more brittle the clay becomes, and the closer to over firing you get. Cone 1 or 2 should be ok for most bodies, the only reason I would advise against it is if you dislike the brownish color that can come from firing higher. It's worth a shot and you really won't know if you like it until you try it. Another option which you may be aware of is to apply terra sig to the exposed clay of your pot (usually just the foot ring or bottom). This will drastically reduce the absorption rate of the exposed clay and keep the rich orange color of the clay. My .02 - I think 04 is the ideal temp for terra cotta, a well formulated body will be plenty dense at that temperature and if absorption is really a concern using stoneware or porcelain may be a better option. I think terra cotta is perfectly suitable for functional wares as long as the user is aware of the possibility of chipping, etc.
  7. Another way to achieve the same thing as adding powdered clay (if you don't have any) is to evaporate some of the water from the deflocculated slip. A wide, low container will give more surface area to evaporate the water and achieve the desired consistency.
  8. I believe what you are seeing is the grog/sand in the clay. If you look in the lower left of the tile you can see bubbles where the glaze has dripped/pooled. Yes, I agree, thinner mixing and application is better.
  9. https://glazy.org/recipes/20547 Try this glaze. Derek Au has tested it on a brown stoneware and there is a picture in the link. He has tested many cone 6 clears and most of them cloud on a dark body. The determining factor from my point of view seems to be the boron level. This glaze has 2x the amount of boron necessary for cone 6, which will diminish the glaze's durability long term. If you want to see the other glazes he has posted click on his name, his tests are a great resource for us all.
  10. Derek Au and Jake Corboy have tested an extensive number of stains in glaze and clay. The base chemistry can affect the color of certain stains (glazes with magnesium can turn chrome/tin [maroon to pink] stains grey) but this should give you a general idea. Scroll to the bottom of the page to see the different stains. These are all cone 6 oxidation. https://glazy.org/recipes/34778 https://glazy.org/recipes/18705 https://glazy.org/recipes/30094 Jake Corboy's website https://www.jakecorboy.com/all-testing-info This is an excellent matte glaze and there are a number of different stains tested if you scroll to the pictures at the bottom. https://digitalfire.com/4sight/recipes/matte_glaze_base_for_cone_6_121.html
  11. I have also read that in Andrew Martin's book and I have had great experience using Grolleg in casting slip. Another thing to consider is that Grolleg has some potassium (flux) and EPK does not. EPK will require more added flux to reach the same level of vitrification for an equal amount of Grolleg. Not sure if I read this on digitalfire or Martin's book but this way to mix casting slip works well for me: mix your slip, let it sit overnight, measure specific gravity, add water to reach target specific gravity, then add deflocculant to adjust the fluidity of the slip.
  12. You might try contacting potter Samuel Johnson. He had one listed on instagram a while back. His instagram is @samueljohnsonpottery and his website is samuel-johnson.com. Another person to ask is Adam Field.
  13. How are you mixing your clay? Something that has not been mention is that commercial clays are typically pugged in a de-airing pugmill. This alone makes the plastic clay much more dense than if it was mixed to a plastic consistency from dry materials (Soldner mixer). Mixing clay in a slurry and drying out will produced a more thoroughly wetted and denser clay body but it is much more work. If you are mixing in a Soldner style mixer there will be a lot of tiny air pockets in your clay. I have not read this anywhere but I have noticed it from my own experience...maybe others can confirm or deny. Either way, a vacuum de-airing pugmill will produce the densest and most homogenous clay body.
  14. In the meantime, you can read these sources online which I have found to be very helpful. The first two links are articles written by Matt Katz and others. UMF may seem very difficult to understand at first but once you understand the general concepts it will be very helpful in your glaze formulation. I recommend looking at Glazy.org, especially the calculator function. Glazy is great because it tells you what oxides (silica, alumina, sodium, calcium, etc.) and how much of each is being contributed by the glaze materials. The calculator allows you to adjust material amounts and see how the chemistry changes in real time. It can all be very overwhelming but the more you read the more you will understand and have control over your glazes. https://www.ceramicmaterialsworkshop.com/reports--publications.html https://ceramicartsnetwork.org/wp-content/uploads/2008/10/TF_BoroninGlazes_0912.pdf http://help.glazy.org/concepts/analysis/#unity-molecular-formula-umf https://digitalfire.com/4sight/troubleshooting/index.html If you are more of a visual learner, it may be helpful to do a Currie grid test of a glaze you like. More information on that can be found below: https://wiki.glazy.org/t/currie-grids/183
  15. 2-3% lithium carbonate, pearl ash, soda ash, or borax can flux the glaze more (increase melting) and may improve the color response. If you don't seive the glaze after you add the material (just blend well) then you will be left with some crystals which will create some localized fluxing. I would not recommend any of this if you were making functional pots. I understand calculating percentage may be a little complicated since you are starting with a wet glaze. You can either add x dry to a cup of glaze, test, then adjust or calculate the amount of dry material in x amount of wet glaze by measuring the specific gravity. If you look at recipes for texture/sculpture/special effects glazes you will notice trends in the materials used and amounts. This will give you a starting point if you are looking for a particular effect.
  16. Can you give us some more information? Cone? Color? Recipe? Here are some ways to make a glaze matte: Increase alumina, decrease silica, add calcium, add magnesium
  17. @kathleencorcoran I have used Redart sig on bisque, then fired to cone 6 with no problems. This may be different with more refractory clays (ball clay/white sig). Sig is very opaque (I think because of the fine particle size) so you do not need a lot to cover the clay body. You can experiment with different thicknesses but I would try to use it the same way you would on greenware. Since you are pit firing the sig may adhere better to bone dry clay
  18. As Neil and Magnolia said, I believe it is either volatile or soluble fluxes. Some glazes do this more than others but especially those with higher amounts of Neph Sy, Lithium Carbonate, basically any on the alkaline fluxes/feldspars high in sodium, potassium, and lithium.
  19. Glazes will look a little brighter and the body will be more white and glassy using porcelain. These are generalizations and it depends, of course, on the formulation of the body. If you are comparing white stoneware vs porcelain in reduction, transparent glazes will look far better on porcelain. Porcelain is generally less plastic and more thixotropic/more difficult to work with. It's a trade off, workability for whiteness/translucency of the body and brightness of the glazes. I say make some of your work with 25-50 pounds of porcelain, 2 different clay bodies if possible, glaze it, and then assess the results. The only way you will know is to try it. There are many nice white stonewares out there but in my opinion porcelain has something that stoneware does not. Many times the compromise of workability is not sensible but it really depends on the type of work you are making and the glazes you use.
  20. Another thing to consider is the clay memory. Porcelain remembers. Say you remove a leatherhard pot from the mold and put too much weight on one side of it (imagine holding a cup sideways). Even if the pot does not go oval in that moment it may be enough for it to cause warping later on. However I suspect this is a problem with uneven drying, even a little bit can cause significant distortion
  21. Are your pieces warping in drying or firing? Evenly drying will take care of most drying warpage, I think. If your porcelain has a high amount of plasticizer this can require more attention. If your work is warping during the firing >>> Do you mix your own casting slip? How important is translucency to your work? Although changing the form will help I think the pyroplasticity of the porcelain is a more important factor. I think many porcelains are over fluxed to make them more translucent (a broad statement). For example you may be able to reduce the amount of feldspar in your casting slip, maintain 0% porosity, and reduce the pyroplastic deformation of your pots. Another option is adding a filler such as pyrophyllite or alumina. From what I have read potassium feldspars create a more viscous melt than sodium feldspars (custer will deform less than minspar in an equal level of vitrification). If you test/mix your own casting slip you will have much more control over the warpage of your pieces and have to compromise the design of the form less. I'm in over my head, calling on @glazenerd
  22. You might try adding a product called Additive A. It's an organic plasticizer and it is especially useful for extruding. I have not used it but from what I have read, it increases plasticity without increasing the water of plasticity. There is plenty of information online about it and many suppliers carry it.
  23. Are you casting or applying slip to thrown pots? I have never heard of using deflocculated (Darvan) slip on leatherhard pots, only for slip casting
  24. I would also suggest starting solid and carving away. If you want an angular form like that use a cut off wire or straight edged trimming tool to slice away. You can go at the block of clay with pretty much anything but I think it will be easier if you have some variation of the form in mind before you start cutting. Also! Look at Tim Rowan's work
  25. I read recently (I think @glazenerd was citing someone else) that with improved grinding methods there is not that much difference between 200 mesh and 325 mesh. For glazes you want 200 mesh or 325 mesh. I think 200 is more versatile and the standard, if you have to choose one. The higher the mesh for any material, the smaller the grain size and the greater the surface area. For most (if not all?) materials this means they can melt more easily
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