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Harold Roberts

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About Harold Roberts

  • Birthday 11/30/1954

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    http://coryellclayworks.com

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    Hallowell, Maine

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  1. This is a clear glaze that I use, fairly simple and if you add 8% rutile it becomes a beautiful white gloss. Oyster Gloss EPK –--------------------- 6 Dolomite –------------- 12 Gerstley Borate ----- 23 Flint –-------------------- 27 Nephelene Syenite - 32 And another clear using less Gerstley Borate. Gloss EPK –----------------------- 6 Dolomite –-------------- 12 Gerstley Borate –----- 12 Flint –--------------------- 30 Nephelene Syenite -- 40
  2. I used to use Frit 3134 as a substitute for Gerstley Borate back in the 80's when there was a Gerstley Borate shortage. I found it flocked my glazes the same way Gerstley Borate did. I concluded that there must be enough free Boron because of the low Silica and Alumina content in the formula that it remained soluble. Boron is a glass former. Anyone who has thrown Borax on a raku pot and found it created a beautiful glassy glaze has seen it in action. Overtime the glaze will desolve in water alone. I have a potter friend who used a beautiful clear glaze she was given by a well known ceramist that had way to much boron in it and overtime her glazes deteriorated. To answer your question Frit 3134 is not a complete glaze. It is used as a flux and needs more silica otherwise the free Boron will soften the glaze. There are a lot of great cone 6 clear glazes out there and davidh4976 glaze is probably a good one. Frit 3195 is much closer to a complete glaze.
  3. I think Min is correct that this is a microcrystalline glaze and that's what is causing the dappled effect and the satin finish. I find it very useful to use a strong magnifying glass to see into the glazes and actually see the microcrystals. Often decreasing the microcrystals in the glaze will strengthen the color. That can sometimes be accomplished by firing higher or by adding more flux to the glaze. Sometimes microcrystals can have a color of their own. The presence of titanium will often give a silvery look to microcrystals but the presence of iron will make them copper or golden. These are good things to understand and maybe guide you in your experiments even though you are mixing commercial glazes which are hard to know what ingredients they may contain.
  4. I find that Wolastonite as a raw ingredient for adding calcium and silica will promote micro-crystals better than using Whiting and Flint.
  5. I have found that water chemistry can much affect clay slips jelling and thinning. Best not to increase your deflocculant. I have found that just a teaspoon or two of common household lye in five gallons of slip will thin out your slip and much alleviate jelling.
  6. Just saw this post and I would add that excess water could well be the problem. I glaze all but my very large pots green, that means I don't bisque them. I use a clay body that has no grog in it and no fireclay. Porcelains and slipcast pots work well in once fired pottery because the are very tight and don't absorb water quickly. I have to be very careful when I apply my glazes because I can get the blow outs you describe.
  7. Hi Betty, After looking at my most recent notes I realize I use Pioneer Kaolin instead of EPK. For many years I used EPK and the reason to use Pioneer was to get a whiter fired look. Another trick I use if the water is hard is to add a teaspoon of household lye. It will make the slip much more fluid. Also you can use up to 20 lbs of water. 16 lbs equals 2 gallons if you prefer to measure it that way and just add no more than 4 more pints.
  8. Hi Betty, For a couple of decades I have used a cone 6 white firing slip that I have never had trouble with. No cracking or warping or glaze fit problems. I always made 80 lbs in two 5 gal. buckets so this is the recipe, Water ............... 16 to 18 lbs with 5 Tbs Davan #7 Ball clay .............3 lbs EPK ..................15 lbs Flint 325 ........10 lbs Neph Sy ......... 12 lbs
  9. I am firing at cone 6 in an electric kiln. I make my own clay for many reasons. Most of the clay bodies available have way to much manganese and therefore very dark. Manganese makes a very cold dark brown and is poisonous. I was looking for a warm rich orange brown look. For that I chose iron magnetite. Also I found that the commercial blends do not have good glaze fit for my glazes. I also raw glaze (no bisque) and need a very tight body without any coarse materials in it like grog or fireclay that would absorb excess water to rapidly. I say this to warn you about why my claybody is formulated the way it is but it is a good starting point and one can add grog or substitute fireclay for the #6 tile. Cone 6 stoneware #6 Tile ..................24 lbs Ball clay .............. 24 lbs Goldart clay ..... 20 lbs Redart clay ,,,,,,,,, 8 lbs Flint ,,,,,,,,,,,,,,,,,,,, 12 lbs Feldspar ............ 12 lbs Iron magnetite ...120 grams
  10. Back to the topic of crawling, I single fire most of the time and have had trouble with crawling especially on overlapping glazes even if I let the glaze dry out before overlapping. I believe it is the dry dusty surface of some of my glazes. I now add dextrin to my glazes and no longer have those troubles.
  11. Just to chime in , all opacifiers, ie tin, titanium, rutile, and zircopax have different effects on colors as do other materials. For example in both photos are the same glazes but the salmon color is opacified with titanium and the white to pink is opacified with rutile. Chrome oxide is the colorant. I imagine that tin would be pinker but less variegated. Also if the glaze was higher in kaolin it might be pinker because chrome pinks love high alumina glazes. It is also interesting how the copper glaze applied to the leaf motif is also affected.
  12. Jumping back a bit to glazenerd's suggestion, granular magnetite, since it is a form of iron does it's color reflect a warmer iron color than granular manganese which is a blackened brown. I have used granular Illmenite but it produces very fine but very warm iron specks.
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