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Post by Arnie Benton on Jul 25, 2015 13:44:17 GMT -6
I'm always tinkering with pieces I don't like - here are 2 before and after - Before After Before After I stuck them into a bisque firing - cone 07 - I guess we can call this hi fire striking - it hid the extensive crazing in the first one but left the crack lines in the second - when the opportunity comes up I'll put similarly glazed pieces into a regular striking and compare - Arnie
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Post by tileman2 on Jul 25, 2015 14:16:06 GMT -6
Arnie:
Guess I am going to have to step down as the king of experimentation: you are blowing right by me. The cobalt blue (2nd) does it for me.
Tom
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Post by mohawkpiper on Jul 25, 2015 20:03:07 GMT -6
Arnie, I once had some stuff under-reduce that I liked apart from the color. I didn't want to refire completely so I put em back in the kiln, took it up to 2010*F (where I was reducing) and then reduced again. To see if I could just re-reduce, like you can with post lowfire reduction. I did not use a cone since I reduce at temps, not cones, because I normally reduce on the way down. Anyhow, I guess it is essentially a super high bisque maybe? But with reduction... I don't know if the reduction did much but here are some before and afters... The purples are before, the white is the same pot but post 2010*F reduced (and same side for the most part). Normally they come out purple, but much darker than they are seen here. This glaze crazes like no tomorrow, but you can't see it since it is so dark so I never bothered with it. The really high struck ones... the glaze just barely started to remelt. It healed most of the crazing. The crystals in the 'afters' are extremely 'optically active.' Almost like a hologram. They are not so much in the befores. G
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Post by Arnie Benton on Jul 27, 2015 16:05:46 GMT -6
Hi Greg -
I've never done a post fire reduction above about 1500 degrees. At 2000 I would have thought there'd be more disruption of the glaze. During the firing, reduction of Cu above 1700 gives craters and otherwise makes a mess of the glaze, in my experience. Ti is deep purple - so it doesn't look like your pieces were reduced the second time and actually lost the purple color - as happens with striking at about 1700. And I thought I was being daring by putting pieces into a bisque - about 1800 degrees.
Arnie
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Post by Arnie Benton on Jul 27, 2015 16:27:05 GMT -6
Here's my latest - I'm trying to reduce crazing - additions to the glaze haven't accomplished much but an engobe applied before bisque firing has a 100% success rate so far. Here's an example - same glaze, with and without engobe.
No engobe - With engobe The good - no crazing. The bad - major changes to the glaze result - Arnie
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Post by Arnie Benton on Jul 28, 2015 15:20:20 GMT -6
And here's what striking did to the pieces above - no engobe with engobe The struck piece with the engobe is a little strange but I like it. Ya never really know what's going to happen. Arnie
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Kuba
Full Member
SztukKilka in Old Formu
Posts: 111
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Post by Kuba on Jul 31, 2015 7:14:56 GMT -6
It would be interesting to see, for example, three pieces with same glaze fired in same schedules. What I have in mind is to check the repeatability of the process
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Post by Arnie Benton on Jul 31, 2015 8:52:47 GMT -6
Hi Kuba -
Cooking in the kiln right now I've got the same glaze on 5 pieces - original clay and 4 variations of engobe used above. And repeated with 2 more glazes. Hopefully the results will be consistent enough to tell us something.
You and Holly were writing about the experience of opening the kiln - tomorrow I get to do it once again.
Arnie
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Post by hollymckeen on Jul 31, 2015 11:14:10 GMT -6
So the engobe result looks totally opacified, as tho there was an addition of 6-7% titanium (which also stops my crazing). So I wonder what was in the engobe that behaved as an opacifier? Usually I look to cut the crazing back in ways that don't alter the clear base... I always seem to get to where it looks great, until is one or two big craze lines show up a few hours later(which totally sucks). But haven't ever gotten all the way gone - but I mostly work with 413 on my clear bases, and I've heard others say it's almost impossible to get rid of the crazing in that formulation. Anyone prove me wrong on that?
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Post by Arnie Benton on Jul 31, 2015 13:01:24 GMT -6
Hi Holly -
The pieces pictured above have a 413 glaze. The engobe has 5% Ti. When I get the results from the stuff in the kiln right now I'll have more info.
Arnie
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Post by tileman2 on Jul 31, 2015 15:27:47 GMT -6
Arnie:
I remember this topic of crazing and followed it on the old forum. I bought this up before: but I have always noticed concentric glazing rings near the base of the plate. From there, it takes on the customary crazing patterns up the sides. I have always wondered if the glaze was pooling where the sidewalls meet the flat area of the plate. Also wondered if the clay was thicker in this area than the sidewalls. I try not to read too much into posted pictures; they can be deceiving in here. In the old forum was sure I saw throwing rings; just turned out to be high opacity reflecting the flash. Safe to assume the engobe in the standard garden variety engobe?
Have some thoughts about this issue; will save them for another day. Tom
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Post by sherri on Aug 1, 2015 12:02:51 GMT -6
Hi Dr. Arnie, Thank you for posting these. I just love the tests especially when there are nice results. I didn't have good luck yet with striking - I put 2 to 4 different glazes on each pot and when I struck at 1350 one of my color combinations on every single one of my 8 pots that were in the firing no longer went together! Which I know means more testing necessary but yikes what a dissapointment that was! But now I know that it makes most colors less saturated except for cobalt and nickel - I'm trying to remember if there were any other colorants that weren't affected at that 1350 temp., I'll try it again eventually.
Greg - Speaking of reductions: I saw this photo of a piece where the cracks went through the crystals - really beautiful. It said that it was the result of a high, hard reduction. I think they might have said that they rubbed manganese in the cracks but it wasn't explained beyond that. I mean, the only thing I can think of is that after the first firing, the person rubbed the manganese in the cracks and then did a second firing. I'm telling you that photo was gorgeous! I have heard of reductions at 1850 and 2000 degress fahrenheit. I've been curious to try it in my electric kiln with my alcohol reduction but am a bit worried about the elements. Do you do that? However, I also read that copper is "killed" at 1750 degrees fahrenheit in a reduction which wasn't explained in a post from the archives. Now I'm seeing your examples Greg which show that loss of color. However, Mariscol has posted many photos of pots he said were reduced at 1850 some of which had copper in them I believe, and the results of course from him were extraordinary and they were extraordinaryly colorful. So, is this how it works or am I way off? - If you do reduction on the way down as part of the first firing there is no loss of color, and if you do a high temp reduction as a separate firing then there is loss of color?
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Post by Arnie Benton on Aug 1, 2015 14:32:47 GMT -6
Hi Sherri - I agree with you. I just looked over the before and after pictures of my last firing and striking. There are only 2 where I think the striking made a significant improvement, and some didn't look as good. I'm trying to figure out which ones really change dramatically for the better - 2 glazes - a bottom glaze with lots of Ti/Ru/IL with a top glaze of Cu Co and Ti or Ni Cu Fe Ti make dramatic changes - about 50 percent of each glaze. I've never been able to reduce Cu above 1700 degrees without it making craters, bubbles, sharp edges and other distortions. But I don't know if I ever tried Cu in combination with Fe, Ni, Mn, Ti, etc. Here's a bowl that I've thought must be crazed, but there aren't any craze marks in the ground, so I've never been sure. Don't know what else it can be. Arnie
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Post by tileman2 on Aug 1, 2015 17:22:31 GMT -6
Arnie:
Can I ask what kind of clay you are using?
Tom
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Post by Arnie Benton on Aug 1, 2015 20:40:26 GMT -6
Hi Tom -
I was using a porcelain clay from Sheffield Pottery in Mass., called 3350. They ran out of some ingredient and had to change the recipe. I switched to another, called Elaine's, and it crazes a lot - I think many porcelain recipes have had to change recently because of loss of a key ingredient - G 200 HP feldspar.
So I've been trying porcelain clay engobes more - I can put them over white stoneware and get the same results as with porcelain clay. The piece I just posted with the crazing crystals is white stoneware with a porcelain engobe - unfortunately the change in clay recipe has forced me to 'start all over again'.
Arnie
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