November 3rd, 2012

December 2014, by PNH

Followup to Murphy: some success, some Murphy.

Yesterday I fired several glaze tests, one of which ends up being a mildly silly story: there is a potter in England named John Harlow. He has a Rutile Blue glaze that he calls Opal Blue, and he provides a recipe on his page of glazes. Unfortunately, it calls for “Wood Ash” which would be wildly unspecified even if he said whether he washes it; “China Clay”, which probably means something fairly clean, as for example Grolleg Kaolin, though there’s no way to be sure without asking him, which I haven’t done yet; and Hymod AT Ball Clay, which is probably quite nice but is not easily available in the US, at least as far as I know.

“Well,” sez I, “I can finesse that.”

First, I just copied the recipe. Tony Hansen provides an analysis of Hymod AT at, so although I had to use Insight’s generic Kaolin analysis and one of its wood ash analyses (Applewood Ash), I was able to get moderately close to an analysis of Harlow’s original glaze. It was slightly off, but there’s only 3% ash in the recipe in any case, so the difference probably wouldn’t have been profound. (Besides, the wood ash that I have is mostly oak, and is probably different from both apple and whatever Harlow uses.) Then I rewrote the recipe, using OM-4 Ball Clay and Sapphire Kaolin instead of the English ball clay and who-knows-what kaolin. At this point I should have adjusted the amounts of the various materials to get as close to the original as I could.

I am, however, a tweak.

Not only am I a tweak, I have a porcelain-person’s uneasiness about sodium, so I rarely use Nepheline Syenite, which is the Feldspar-equivalent material in the original recipe. I rewrote the recipe to use G200HP Feldspar, which is my regular Spar these days. That obliged me to rebalance things a bit, and I ended up without any Kaolin at all. I also ended up using Bone Ash (we get synthetic stuff these days, so it is fairly uniform) instead of Wood Ash. At that point I had, almost needless to say, deviated significantly from the analysis of the original, so I wasn’t really sure what I would get, but I was on a roll, so I mixed up a small batch and fired a test tile on Thursday. It came out looking like this:

Although this is not exactly a Rutile Blue, I think it’s a keeper. It is probably too plain to use all by itself, but perhaps with some iron or cobalt brushwork (which I will have to learn to do, but that’s how it goes).

Murphy hit: this afternoon I fired the kiln again. During the firing, a glaze test fell over against an otherwise rather nice Rutile Blue teacup, sticking to it and ruining it. After I knocked the glaze test off the cup I ground down the sharp junk it left behind, and someone at the party here this evening asked for the cup before I could even get a chance to photograph it. (I would not have taken it to Toronto anyway, as I can only bring a few pieces, and I don’t really want too many of them to be examples of badness.)

Meanwhile: on Thursday I had fired another Rutile Blue test in addition to the creamy thing I show above. This one was a followon to one of my own, from a series I was working with a few years ago; I changed two or three of the materials in it, and I expected to get something fairly reasonable, but instead it fired out “Southwest Red’, just as FOB12 did a few weeks ago. (See “...In Which We Learn by Resounding Failure”, posted on September 20th.) I thought about that a bit, added one gram of Rutile, and dipped a second test tile, which I fired this afternoon. Here they are:

(Some of you will notice the fact that I have run out of test tiles, and am using broken pieces of teacup from recent failed bisque firings. Argh.)

The one with extra Rutile appears to be a keeper, and if it does not exhibit any wretched surprise behaviors it will probably replace my recent batch of Rutile Blue, which is better than the mess that preceded it, but has not been quite as well-behaved as I’d like. Speaking of which, I still haven’t managed to get a recipe to fire out like the dark blue mixture of two recipes that I posted a while ago —

— and I am somewhat peeved about that. At this point I’m not even sure I would get this result if I mixed new batches of the two parent glazes and poured them together again; I will have to try that as time and tide permit, but it probably won’t happen for a while.

I am also having some trouble with the copper red glaze, but it is still a deep rich red even though there is only 0.1% copper oxide in the recipe. (I will grant that I am using red copper oxide, which has more copper in it than black copper oxide, but still, 0.1% is not a whole lot.) When it is behaving itself, that glaze looks about like this:

I am still hoping to bring a piece with this glaze (or a close variant) to Toronto, but as I say I’ve been having some bad behavior from various versions that I’ve fired recently; we’ll have to see how it goes.