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Post by Arnie Benton on Feb 25, 2017 11:35:07 GMT -6
I'm trying to put together some info on the effects of striking at different temperatures. Hopefully I can provide some data that will be of use to those of you who have more experience and especially knowledge of what goes on with seemingly solid structures to produce major changes in colors. Maybe it has to do with changing magnetic forces, maybe with inflection points of alpha to beta willemite, maybe I don't know what I'm talking about, maybe, maybe.
But I am fascinated by the results, and may end up just going with what I like - but the scientist in me would be happier with some theories - which could then be tested further.
I made 5 sets of glazes and put each on 3 pieces - 15 pieces in all, fired in the same kiln at the same time in my usual crystalline firing - then I divided in thirds and put one of each set in a 1300F striking, a 1350 striking or a 1400 striking.
Here's what the typical piece looks like after the crystalline firing -
After 1300 striking -
Same glaze, different piece, after 1350 striking
Same glaze, different piece, after 1400 striking
The effect of increasing temperatures is much the same in each of the other 4 sets - 1350 F produces the white splashes and 1400 increases the white, decreases the blue and adds a rim of deep brown.
I have other examples with Fe, Ni, Ru instead of Ti, no Cu - and will happily provide pictures to answer specific questions, but I think more at this point would just be information overload.
This crystalline firing produced fewer crystals than I expected, so I'll change the firing schedule, add more Zinc the next time - but more ground does help to see what's going on.
Arnie
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Post by tileman2 on Feb 25, 2017 21:43:07 GMT -6
Hi Arnie:
Having not really studied the chemical processes involved in a strike firing: my initial thought would be selective corrosion: also known as selective demetalification. Used by industry to purify metals; by using annealing temperatures (1250-1500F) to remove selected metals from solids that contain several. It gets into ionization energies, reduction potentials, and a couple of thermodynamic laws. Your initial firing shows the higher presence of cobalt (CO), and each successive strike firing picture shows less and less of cobalt as the temps raise. By heating the piece up to the point that the metal oxides are softened, but not molten: you are accelerating what oxygen does to metals at ambient temps. Cobalt, copper, iron, and zinc are all subject to natural oxidation as part of corrosion. Titanium however is resistant to corrosion by naturally occurring oxidation. Iron is Fe, and as it is oxidized (corrosion) it becomes FeO2 0r 3. In your initial firing, the color is produced by FE, and as you strike fire it is returning to FeO2 (3). The oxidation changes the color; it becomes lighter and lighter. Same for copper, cobalt, and zinc. You will notice in all your examples the colors produced by cobalt, copper, and iron: all soften. Titanium however resists corrosion: so the pieces get whiter and whiter. I think you are also dealing with what is known as the miscibility gap: a temperature range where solids are suspended between two solid state phases.
Study this: Intergranular corrosion
Tom
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Post by Bernhard Schärf on Feb 26, 2017 6:54:20 GMT -6
Whether piezoelectricity (electromagnetic waves) - Jim's thought, or intergranular corrosion - Tom's thought .... or alpha / beta-willemite .... My suggestion is the following 2 attempts: 1. burn the last pieces (after 1400 striking) again until 1650. 2. typical crystalline firing (the first piece), but rapid cooling to 1400 and then hold for 2 hours. Only such an idea ? Bernhard
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Post by jfox on Feb 26, 2017 10:10:46 GMT -6
I dont think polarization has anything to do with it, i was just commenting on the interesting properties of titanates I think Tom is on the right track with oxidation states clearly some kind phase changes going on. ive been obsessed with the single fire process so ive wondered just how cool you have to go before you can strike? perhaps just holding at that temp might do it or what is the max temp of cooling required? perhaps that would indicate a phase change of some sort
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Post by Arnie Benton on Feb 26, 2017 11:16:17 GMT -6
Hi Tom - I think your ideas are most interesting. I have certainly noticed the 'faded' look, especially with Co - The pictures shown above don't have any Fe in the glaze - only Ti Cu Co - I don't understand where the brown color comes from.
Hi Bernhard - I can do both of your suggestions -
Hi Jim - I do know that 'post fire reduction' looks much different than 'reduction while cooling from a crystalline firing' - and have thought that the difference came from cooling to room temp and then reheating the post fired pieces.
I appreciate the feed back. It gives me a new burst of energy!
Arnie
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Post by tileman2 on Feb 26, 2017 20:22:57 GMT -6
Arnie: some tests I can recommend that will help prove/disprove the corrosion/oxidation theory. Tin is likewise used with metals to hinder corrosion; just as titanium is. From the color, I assume you are using cobalt carb?.. or a very small amount of ox.
Tile 1. 1% CO. carb. % 1% tin Tile 2. 1% CO carb. & 2% tin Tile 3. 1% CO carb. & 3% tin. Make four of each: one control piece, and one each for each of your firing temps. Same protocol you are using now. Watch and note the color of the cobalt: does it fade as before-or not?
Test 2. Make your normal layered glaze configuration: except use only 0.25% titanium: just enough to seed. Fire the four according to your testing schedule: will prove/disprove the white colors.
The dark yellow/tan/ golden brown color could be from iron; if you are using stoneware. It could also be what is known as brookite: if your TiO2 is made from natural titanium sources (most is not). Brookite is not the uncommon in high iron/titanium stoneware bodies: producing the same colors in the clay.
Additional thoughts: crystals are an ionic bond: most all other glazes are a covelant bond. An ionic bond is produced by bond polarity; isoelectric points: band gaps-etc. If the strike firing was effecting band gap; then the crystals themselves would become malformed because the electrostatic charges holding the ionic bonds together would disassociate. None of your samples show signs of the crystal formations themselves showing disruption: so that sorta takes the electrostatic molecular forces being altered off the list.
Crystals are a zinc-silicate formation: Zno2Sio4 = willimete. If the heat was oxidizing (selective corrosion); this would also cause decay/disruption of the crystal formation: so I would take zinc/silica alterations off the list as well.
These 2 tests should help you along the way..happy cooking. Tom
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Post by tileman2 on Feb 26, 2017 20:27:43 GMT -6
By the way: additional notes. In the glass arts: 1350F is used in glass slumping. 1450-1475F is used for fusing two pieces together. I point this out only to note that the glass/glaze itself is hot enough to open it up for porosity. This would allow conduction of oxygen present in the kiln.
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Post by Arnie Benton on Feb 27, 2017 10:24:39 GMT -6
Tom - look like great suggestions - My clay is a pure porcelain, so I don't think that's where any Iron is coming from. Here is some additional info - the same piece struck at 1350, 1450 and then 1650 F. Even at 1650 the crystals seem the same, to me, except for the loss of the Co color. The glaze has Fe, Mn, Ti, CaCO3 on the bottom and Cu Co Ti on the top. My usual glaze has .5% CoCO3. struck at 1350 - struck at 1450 struck at 1650 I don't have the 'before' picture for this one, but it looks almost like the original in the other series except the crystals have lost their Co color. Arnie
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Post by Bernhard Schärf on Feb 27, 2017 10:53:36 GMT -6
struck at 1650 ...... but it looks almost like the original . Arnie Thanks Arnie, that was exactly my guess that the blue background comes back ...... I will stick with my theory that the beta-willemite (micro) have changed back to alpha-willemite. Bernhard
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Post by mohawkpiper on Feb 27, 2017 15:25:11 GMT -6
Hey Arnie, I have been doing similar striking experiments for some time and whereas i dont have all the answers ill share some of my findings.
Striking to a cone is more accurate than to a temp. I can get the same look on the same glaze at 1270 and at 1360 F depending on my rate of climb and if there is a hold or not as long as i hit the same cone. cobalt always seems to turn a faded pastel blue more so the higher you go, for me starting around cone 016. others that strike well are titanium and copper( copper usually just darkening) and will do by themselves, iron will strike but its much more picky and usually will only do so in conjunction with others. i think manganese might be like iron but hard to tell from the experiments ive done. havent played with that one too much.
you can always restrike up but not down. so if you strike a piece to cone 018 and another of the same glaze to cone 016, then restrike the 018 up to 016 in another strike, they will look the same. but if you restrike the 016 down to 018 it will still look like 016. So you can essentially restrike just one piece multiple times to see what it would look like struck at different cones.
its like an additive kinda thing i guess.
from my experiments it seems striking works well between cone 018 and 011 and above that it starts to wash out up until about cone 02 everything i have done just turns white or some form of it in both the bg and crystal with the exception of some glazes in the thinner areas getting a streaky darkish color.
G
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Post by mohawkpiper on Feb 27, 2017 15:41:12 GMT -6
and about how cool you can go... i dont have an answer to that either but i have cooled to about 275 then took the kiln right back up and gotten the striking effect to work without even opening the kiln. used glazes i already knew what they looked like both struck and unstruck so i know it worked as it should have.
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Post by Arnie Benton on Feb 27, 2017 17:06:57 GMT -6
Thanks Bernhard -
Beta willemite changes to alpha with increasing heat - question is whether alpha changes to beta as temp cools - my guess in that it doesn't, in agreement with what Greg is writing above.
Hi Greg - I haven't been using cones - but my heating program is constant, so I think my results are pretty consistent - I will add some cones and see for sure. Have you gotten anything that's similar to the 'splotchy' ground in the pictures above? I don't know how much it has to do with vertical and horizontal surfaces and the layering of 2 glazes.
Arnie
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Post by tileman2 on Feb 27, 2017 17:58:58 GMT -6
Little confused by the alpha/beta willemite reference: was not aware that it did. Willemite is zinc/silicare: and crystals are similar in composition. Silica undergoes alpha to beta at 573C; but have not read anything in regards to zinc doing the same. To test effectively, you are going to have to reduce the number of oxides down to one at a time. with the multiplies you have going on now; would be nearly impossible to determine which one is doing what.
What I see in the 1650F piece is covelant and ionic bonding differences. Ionic bonds (crystals) are formed by electrostatic molecular forces: band gaps primarily (Ev electron volts). Paulings scale of electronegativity would be applicable. Only the crystals are ionic bonds, the field colors are covalent bonds: shared molecules. The crystals ejected the transitional metals because of the bond polarities holding the colorant in the crystal was disrupted. However, covalent bonds are not disrupted by changes in electrostatic polarities: so the transitional metals would still share molecules: like any other glaze used in ceramics. Glaze eutectics would be applicable to the field color, but not the crystals.
So Arnie; you have a lot of testing ahead of you at this point. I would start with the titanium first; study the effects of increasing amounts up to the point of your normal glaze additions. Then come back and hit one colorant oxide at a time, noting the effects of each. I will make an educated guess of 1300F minimum for striking: because the glaze has to become porous enough to allow for oxidation. If you are looking for the lowest temp, before you climb back up to strike temps: I would go with 573C: the temp at which alpha quartz becomes beta.
Tom
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Post by jfox on Feb 27, 2017 20:32:44 GMT -6
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Post by Bernhard Schärf on Feb 28, 2017 3:24:50 GMT -6
- question is whether alpha changes to beta as temp cools - my guess in that it doesn't, in agreement with what Greg is writing above. Arnie Yes Arnie, alpha-willemite are the more stable form and will not be converted back into beta-willemite. You can see it clearly on all your attempts that the alpha-willemite in the structure have not changed. My second proposal would be of interest to me whether a cooling down to room temperature with repeated heating up to 1400 is necessary, or whether these effects can be produced even in the same fire. A further suggestion would also be a microscopic examination, whether it is amorphous or crystalline structures. It could also be primary particles of Zn / Si mixed oxide. (E.g., ZnO nanocrystals and amorphous SiO2). Bernhard
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