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The Newsletter for Raku Artists and Raku Art Lovers In This Issue ~-~-~-~-~-~-~-~-~-~-~-~-~-~-~-~-~-~-~-~-~-~-~-~-~-~-~-~-~-~ Yeah, I know. It's not May any more. Yes, this newsletter is WAY late, but I figure better late than never, right? I've changed the format of the newsletter a little bit, which I think will help me get the issues out a little easier and faster. Plus you should be able to tell what's in each issue a little easier as well. Let me know what you think. One thing I wanted to make sure and tell you is CafePress is having a sale. You get $10 off any order of $30 or more until Jun 14, 2005. So hurry! Just use coupon code GRDAD05 (that is a zero-five) during checkout at any of my stores: Just Raku Logowear http://www.cafepress.com/justraku I appreciate your support - plus you get to save some coin. As always, I look forward to your feedback, suggestions, questions, and
article ideas at: Gary R. Ferguson ~-~-~-~-~-~-~-~-~-~-~-~-~-~-~-~-~-~-~-~-~-~-~-~-~-~-~-~-~-~ * Fading Glazes Feedback * Chris R. had the following feedback on the issue of fading colors of
some Raku Glazes. The rainbow colors on a copper-matt pot are caused by firstly reducing the metal oxides in the glaze back to metal and, secondly, allowing that shiny metal surface to oxidize slightly, which produces the light interference that gives us the color effect we seek. If the exposed metal surface is not protected, then it will be subject to further oxidation - in the case of copper, it may even oxidize all the way back to black copper oxide, as your contributor has seen. In other Raku glazes, the overall result is a porous ultra-thin layer of colored glass on or near the glaze surface whose complexes are open to attack by airborne oxygen, moisture, and sulfur compounds. Put simply, the reduced glaze is too porous and/or too easily affected by the environment. The original Raku glaze cut-and-try approach led to this problem, since good glass does not come from impure calcium borate (Colemanite/ Gerstley borate) and/or hydrated sodium borate (borax). Neither one of these borate materials by itself, nor any combination of the two together, will form a long-lasting surface. Also, it takes more than 20% Nepheline Syenite to provide enough silica and Alumina to make the glaze durable. A look at the Seger formulas for most Raku glazes shows why they may change. They often lack sufficient silica (quartz, flint or, chemically, silicon oxide, SiO2) to form a durable glass. Many are also deficient in alumina (aluminum oxide, Al2O3), which toughens glass and helps to seal the glass surface. Even Branfman's book http://www.garyrferguson.com/branfman gives the same "80/20" borate glaze recipes of the 1960s (with minor changes) although, to his credit, he also lists newer balanced glazes that use frits in their mix (both borate and alkaline frits). So what is the answer to the fading problem? Seal the glaze, either by
using a varnish of some sort (but keep it out of sunlight) or by making
the glaze a viable glass by a combination of intelligent alteration of the
glaze chemistry and LOTS of testing. Glaze chemistry is fascinating and,
once the initial concepts are grasped, not too difficult to understand.
Learn about it now. You'll have to when the Gerstley Borate runs out
anyway! ~-~-~-~-~-~-~-~-~-~-~-~-~-~-~-~-~-~-~-~-~-~-~-~-~-~-~-~-~-~ Part I Part
II Part III |
