Comment

Comment by Norm Stuart on March 21, 2013 at 11:02pm

We originally held for 20 minutes to let glazes lay flat, but I was intrigued with idea that we weren't giving Brownian Motion sufficient time to complete reactions in a viscous melt, so we started holding 60 minutes. Another idea was that a one hour hold, by definition creates the heat work of the next higher cone (or so I've read at Orton) so we could better densify some of our clays without actually reaching a higher peak temperature which would decrease the viscosity of our glaze melts, many of which run badly at cone 7.

I've recently been comparing 30 minute holds at the top to 60 minutes, as I did for this Orange Street glaze.

I've previously tried George Lewter's hour holds at about 962C, which improved glaze using the Laguna Red Iron Oxide, which is partly not red iron oxide. But it did not compare to using pure Red Iron Oxide.

I found a note that John Britt cites 1,239C (cone 7ish) as the point where Red Iron Oxide converts to Black Iron Oxide, the link below says it begins at 1,220C, so a cooling hold from Cone 10 would be important. But it seems simpler to maintain the Red Iron Oxide used as the glaze ingredient by not reaching 1,239C.

http://books.google.com/books?id=TApnGTVLwxAC&pg=PA189&lpg=...

But this is also a function of time, where reactions require a certain amount of kilocalories / mole of Red Iron Oxide, the number Google doesn't seem to yield at the moment. For this reason I cut back the 60 minute hold at 1,222 to 30 minute in case it begins to diminish red iron oxide.

Deciding what to start with rather than a mixed powder like Laguna red/black also makes sense since Red Iron Oxide is refractory but Black Iron Oxide is a flux above Cone 010.

I look forward to your photo.

Comment by Jeff Poulter on March 21, 2013 at 9:33pm

I will snap a pic tomorrow. I fire to ^6 with a 5 min hold. I slow cool at 50•c/hr to 962•c and then hold for an hour.

Comment by Norm Stuart on March 21, 2013 at 9:07pm

If you look at the bottom of the tile on the left, you'll see two tiles are made of white clay, specifically Laguna Stoney White. http://www.lagunaclay.com/clays/western/wc412.php The tile on the left fired vertically and the one on the right horizontally with very little difference in appearance.

Bear in mind the kiln fired periodically to achieve a 50 degree per hour cool between 1,800 and 1,500 which makes glazes more crystallized and matte. But glossy glazes still come out glossy.

Can you post a photo of your tile?

This is a two layer Oil Spot on a vertical tile which slid during the melt exposing areas of the tile.

Comment by Jeff Poulter on March 21, 2013 at 8:43pm

My tests came out more almost glossy than semi-matt. Of course this was on a heavy iron clay shard. Has anybody tested on white clay or porcelain? I like it well enough that I'm going to mix some up on Saturday and spray it on a pot. It is really a nice glaze. I will be testing it with some different subs on the yellow iron oxide and also compare the special RIO as opposed to the precipitate. Jhp

Comment by Norm Stuart on March 21, 2013 at 5:12pm

I used Yellow Iron Oxide, which supposedly just 88% of Red Iron Oxide. The LOI being water, hydroxyls.

I went into this believing the H20 firing off the Fe2O3.H2O is not relevant, and probably isn't. 

But I've already been surprised already that one ingredient or another can make a glaze difference in the relevant Cone 6 range when you're not first firing to Cone 10.  I have an open mind at the moment.

Comment by Jeff Poulter on March 21, 2013 at 3:04pm

I got my RIO Precipitate from standard today so I can mix up another small batch.  I'm wondering if I should try & find my crocus martis or stick with the purple ochre.  I was in a hurry last night & wanted to get the test piece in with the other pot so I didn't look as long as I could have.  I guess I could have gone to the local supply and gotten yellow RIO.  I also have a bag from Ruelev(spelling?) that is an artist supply house.  It is a Red Hematite.  I have done a couple tests in other glazes with it & it makes things come out more reddish-purple.  So many tests, so few test tiles!  

Comment by George Lewter on March 21, 2013 at 9:01am

This has a very nice semi-matte surface. Looking very much like a cone 10 iron red.

Comment by Jeff Poulter on March 20, 2013 at 8:29pm

I have the special iron red oxide from ceramic supply and pricipitate on order from standard. I made up a 100 gram batch using special rio and I subbed purple ochre for the yellow rio. I know I have crocus martis as I have made van gilder red before but I couldn't find it. I have a chip in a firing as we speak so I will see what it looks like by Friday.

Comment by Norm Stuart on March 20, 2013 at 1:05pm

I've found "red iron oxide" comes in a variety of purity. The one from Standard Ceramics beats Laguna hands down as it is pure red iron oxide and doesn't contain barium sulphate. Laguna Red Iron Oxide tends to fire brown to dark brown in our kiln at Cone 6.

Firing to Cone 6, the Red Iron Oxide Precipitate remains stable - except on thin areas of the ware which receive more heat where it breaks down to Black Iron Oxide. I'll be comparing the Standard Ceramics "precipitate" with "Special Red Iron Oxide" from U.S. Pigment.

These tiles were fired to Cone 6, held for 30 minutes and slow-cooled at 50F between 1,800F and 1,500F. A differing firing cycle could change the resulting look of the glaze.

http://www.standardceramic.com/Materials.html

Other posts on this subject:

http://cone6pots.ning.com/photo/img-0380?commentId=2103784%3ACommen...

Comment by Christopher Cisper on March 19, 2013 at 10:11pm

I like this glaze alot!  Is it fairly stable? And how is red iron precipitate different from red Iron oxide?

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