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Post by Arnie Benton on Oct 12, 2015 10:38:32 GMT -6
I've been doing many strike firings - instead of post fire reductions - Here's the same glaze applied to the same shape shallow bowl - one with an engobe applied before bisque firing and the other without the engobe - No engobe -  and then struck -  with engobe -  with engobe and struck -  I continue to be impressed and fascinated by the effect of striking - and the engobe clearly makes a big difference. The engobe gives the background a greenish tint which then leads to a light reddish brown color after striking, which gives a much more lively effect to the piece. The glaze has Fe Cu Co and Ti - I don't pretend to know anything about color development - I just know what I see. Arnie |
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Kuba
Full Member
SztukKilka in Old Formu
Posts: 111
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Post by Kuba on Oct 12, 2015 11:05:40 GMT -6
Arnie, You are the master of testing! P.S. You should write a book, I would definitely buy it!  |
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Post by tileman2 on Oct 12, 2015 19:36:18 GMT -6
Arnie:
I think its time to officially change your name to Arnie Engobe. Okay, let me process and organize my thoughts- and I got nothing. Seriously though- a clay body does not have any TiO2 or whiting (calcium). Your engobe has the amounts shown- which leads me back to leaching. Will guess the Cu is .25%, the Co at .50 to .75%, Fe at 3-5%, and TiO2 we already know is 5%. The engobe piece is showing the classic whitish/grey areas that high amounts of TiO2 leave behind on horizontal pieces. Not so sure the green area in the engobe piece is from Cu alone; high percentages of TiO2 can produce that color: example: high amounts of Rutile, Ilumenite can produce that green cast.
The Co crystal color also lightened in the second piece; which I would attribute to high TiO2. The FE really became highlighted as well. The one thing I cannot guess at or assume is the large area of whitish/grey in the first piece without engobe after striking. Cannot think of a reason it should be there other than the TiO2. If your engobe had 5% of TiO2, how much was in the glaze itself? You also rotated your engobe pics slightly from the first to the second. Your twin crystals are at noon in the first pic, and at 2 o'clock in the second. How is a guy suppose to track subtle changes when you rotate your pieces when you take pics? The large single crystal is at 3' o'clock in the first pic, and at 8 o'clock in the second.
Tom
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Post by Arnie Benton on Oct 13, 2015 9:29:51 GMT -6
Thank you, Kuba!
I did some middle of the night half dreaming half thinking about your idea -
Pottery - Crystalline glazes - horizontal surfaces - multiple firings - reduction - layered glazes - engobes -
how many people do you think there are in the world who would be interested in this level of detail about this tiny corner of the pottery world?
It made me realize how fortunate I am to be able to fire my kilns hundreds of times without having to worry about cost -
Tom -
I rotated the picture -
Rutile and Ilmenite have Fe, which I would guess contributes to the greenish cast.
The pieces above have a glaze with Fe 2 Co .2 Ti 4 on the bottom and Cu .5 Co.5 Ti 2 on top, with a ratio of 25% bottom glaze and 75% top glaze.
I'm going to try in the near future to combine the glazes into one but given what I imagine to be incomplete mixing and uncertain amounts of leaching, I am having trouble figuring out the quantities. My previous attempts at combining the 2 glazes into one by mixing 3/4 of one with 1/4 of the other haven't given the mottling, streaks and color changes of the layered glazes - (most of the time). Sometimes it works, but much more consistent with layered glazes.
Arnie
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Post by tileman2 on Oct 13, 2015 16:11:05 GMT -6
Arnie:
You and I suffer from the same disease: whatifitis. This glaze can be the most frustrating, but it can also be the most expansive.Just when you think you hit the boundaries; new doors open. Keep the inquisitive mind; it will keep the synapses firing. When you get the chance; read some background on cornwall stone- you might find the ingredients and applications interesting for engobe use. Trying to get a friend of mine to send me some pigment grade TiO2. It should not seed given its composition; but it should make for an interesting colorant.
Tom
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Post by Arnie Benton on Oct 14, 2015 9:08:19 GMT -6
Tom -
At this point I'm more into the details of firing schedule, layering of glazes, changes of small magnitude in glaze recipe, striking, reducing,etc - leaving the basic ingredients the same - once I change an ingredient, especially one as complicated as Cornwall Stone, all of the other stuff no longer applies -
Am just starting a new batch of 413 frit - I hope that's not too disruptive - or maybe I'm just getting older or more conservative or smarter.
Is the Ti a finer grind or something else?
Arnie
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Post by tileman2 on Oct 14, 2015 18:58:36 GMT -6
Arnie:
The reason I suggested
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Post by tileman2 on Oct 14, 2015 19:08:52 GMT -6
Arnie:
The reason I suggested cornwall stone is the type of pieces that have unique parameters. CS already has the calcium (whiting) you like to use. It is considered a flux actually, although it has high percentages of SiO2. It is favored in engobes because it has a binder (adhesion) property. You seem to prefer layered glazes I assume for visual effect. Was wondering if CS would help you in achieving the highlights you are after. Your pieces without engobe seem to pool glaze, and the pieces with engobe keep the colorants distributed. Was thinking aloud you might say- wondering if CS would help keep your layered effect in place. I have never heard you mention alumina additions; so again I was assuming you have found the right peak temp that achieves melt right before it gets too fluid. Partly also because of my preference to use nature made materials in lieu of man made chemicals.
Tom
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Post by Arnie Benton on Oct 15, 2015 12:50:18 GMT -6
Hi Tom -
I've been asking myself similar questions about the swirls and color variations in my bowls, especially after striking - are they coming from interactions of the glaze/engobe/clay ingredients or from physical factors like surface tension - hard to make a case for either when the changes are coming at a top temp of 1350 F - no melting of the glaze, so how can such dramatic effects occur? I can better imagine that the layered glazes and interaction of the inner glaze with the clay/engobe can create some different effects - but I would think there would be more evidence of these effects prior to striking. In the kiln right now I've got the same 2 glazes in 5 variations - with and without engobe, mixed together prior to application and with additions of Ti and CaCo3 - the engobe additions -
Arnie
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Post by Arnie Benton on Oct 16, 2015 9:12:27 GMT -6
OK - here's before and after striking 1 - Engobe with layered glazes   2 - No engobe layered glazes -    3 - NO engobe, layered glazes with addition of Ti2 -   4 - No engobe, glazes mixed together prior to application -   5 - No engobe, glazes with Ti 2 added and mixed together prior to application -   As I'm entering this info, piece #2 is cracking loudly. So, #1 engobe with layered glazes gives best crystals, followed by #3 with the engobe additions added to the layered glazes - I'll add more Zinc to the others in the next go around. Clearly, adding Ti2 to the glazes seems to accomplish the same result as the engobe. Arnie |
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Post by tileman2 on Oct 16, 2015 21:26:09 GMT -6
Arnie:
Decided to look for some answers about why strike firing has such dramatic effects. We have discussed annealing before: glass and metal react to annealing. The fundamental property in both cases is to heat just below the temperature that alters the physical structure. So in the case of our glaze: strike firing is softening the glass and the metal oxides. After that, the next phase I will have to read about for some time to come: Polymorphism in binary metal oxides. Will keep this simple: strike firing brings the metal oxides to a temperature where they can lose or gain additional oxygen molecules; which can alter their color. In reading through some of the excessively boring journals: it appears that TiO2 is very reactive to change in this transition. Sorta makes sense though: the color you get from FE in an oxidation firing is different than a reduction firing. When Jim gets done catching up with his firings perhaps he can chime in.
I still wonder about your clay body. It does not seem to be crystal friendly unless you spike it with an engobe cocktail. Looking forward to seeing the strike results. Very curious to see how plate 2 turns out- can already see the TiO2 pooling towards the center. There is a large white patch below the single crystal at 11 o'clock. Interested to see how much that area changes.
Tom
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Post by Arnie Benton on Oct 17, 2015 9:43:46 GMT -6
I put the before and after pictures together in the post above -
It's clear that the engobe piece is the winner - the only one without major cracking and with the most interesting background.
#2, with layered glazes, has changes in color and some movement - It would look better with more crystals.
#3, layered glazes with Ti added shows a different crystal color and a bright red/brown ground.
#4, glazes premixed, has much less change than #2 with layered glazes.
#5, glazes premixed with addition of Ti, is very similar to #3 with layered glazes and the same additions.
With or without the engobe, the layered glazes without additions produce the most color variations with striking. Adding Ti to the glaze causes the loss of the color variations that occurs with striking.
Mixing the glazes together prior to application causes the loss of color changes with striking.
SO, I need to make pieces with the engobe and mixed glazes and with the engobe and glazes with the Ti added to complete the picture -
It's well known that you can strike Cu reduced pieces at 1300 F in a neutral atmosphere and get brighter reds, because the Cu molecules rearrange themselves and become more visible to the eye. So molecules do move about at 1300 degrees - I agree with Tom that there is molecular movement in that temp range and the movement can cause changes in color - I believe that all the glazes I have used which produce these dramatic effects with striking contain some Cu.
Why the same chemicals in an engobe react so differently than when added directly to the glaze maybe has to do with them being more concentrated at the glaze/clay interface??
Arnie
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Post by tileman2 on Oct 17, 2015 15:15:28 GMT -6
Arnie:
Great looking pieces; strike firing certainly achieves your goal of glaze highlights. Been scrolling back and forth between pics for 20 minutes looking at every little detail- I noticed you aligned most of the pics. I could ask a hundred questions and just as many comments about the subtle changes. I see the CU changes, but also the FE, and TiO2 changes. The bronzing of the crystals in pic 3 & 5 have my attention: see the same results in alkaline etching. Is it the CU, FE, or TiO2 causing this dramatic change both in crystal and field color? Perhaps in the future you can run tests by omitting one in each piece. Regardless- those are some mighty fine looking plates.
Tom
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Post by Arnie Benton on Oct 18, 2015 12:41:57 GMT -6
Just put some more combinations in the kiln - engobe with mixed glazes, engobe with Ti addition only in bottom glaze, etc.
Yes, I will try leaving out Fe, Ti, Cu - I'm also curious about leaving out Co -
Arnie
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Post by Tracey Renner on Oct 18, 2015 22:31:29 GMT -6
Hi Arnie, You said that one of your plates was cracking and I have heard you mention a couple breaking....I believe you do not use crystalline glaze on the back of your plates. Does it make it more likely for the plates to break when you use a crystalline on one side and a cone 10 on the other side? Does anyone else know if that impacts plates? You know, they are plates and by definition annoyingly fragile.
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