Lustre / Reduction Chemistry Question

[mousey]

So I've been going down the lustre / raku reduction chamber rabbit hole of late and was curious; what chemical reaction produces the iridescent film itself?

 

Is it simply depriving the environment of oxygen?  Or is it rooted in how the oxygen is consumed/destroyed by flame within the chamber?

 

Was toying with building some sort of reduction chamber that displaced air with an inert gas but it occurred to me that I might be barking up the wrong tree entirely there...

 

 

[glazenerd]

Start here: https://en.wikipedia.org/wiki/Redox

[mousey]

Start here: https://en.wikipedia.org/wiki/Redox

 

 

Ahhhh ok, so oxygen-thirsty burning fuel runs out of places to get oxygen so it ends up pulling it away from the glaze.  Although I'm not entirely sure yet how that translates into the metals coating the surface, it certainly does answer my question about the difference between reduction and an non-oxygenated environment.

 

Thanks very much, I appreciate it.

[glazenerd]

 

https://en.wikipedia.org/wiki/Isomorphism_%28crystallography%29

https://www.ces.ncsu.edu/plymouth/programs/optical.html

[glazenerd]

 

https://en.wikipedia.org/wiki/Anisotropy

[mousey]

 

 

https://en.wikipedia.org/wiki/Anisotropy

 

"Most common rock-forming minerals are anisotropic, including quartz and feldspar."

 

the plot thickens.....

[glazenerd]

 

https://en.wikipedia.org/wiki/Thin-film_interference

[glazenerd]

 

Last one: https://en.wikipedia.org/wiki/Combustion_chemical_vapor_deposition

 

Combustion chemical vapor deposition (CCVD) is a chemical process by which thin film coatings are deposited onto substrates in the open atmosphere.

Which is what Raku is: depositing a thin film onto a glazed piece. The crystal structure is refracting light back through the thin film; creating the optics of iridescence.

[PeterH]

Hi,

 

You might like to try this. You will need:

- A metal canister open at one end a little bigger than your pot.

- Two pieces of chipboard big enough to cover the end of the can.

- A brick or two.

- A newspaper about 1/4 inch thick.

- A bucket of water.

- Rubber bands or string.

- Some charcoal.

- A plastic shot glass or similar.

- Some alcohol (UL meths, US ??)

 

- Place the first piece of chipboard on the ground, with the tin on top. (This insulates the tin a bit.)

- Cover the bottom of the can with charcoal.

- Put the newspaper into the bucket of water to soak.

- When it's getting nearly time to take the pot out of the kiln take the newspaper out of the water.

   Let it drip a bit then wrap it round the chipboard and secure it with the rubber bands or string.

   You should have a nice flat region of paper that will securely seal the lid of the can.

- Fill the shot glass with meths.

- Take the pot out of the kiln, and place it in the tin on top of the charcoal. Keep it away from the sides.

- Rapidly:

  - Pour the meths into the tin. [H&S keep your head well clear, and wear something like a leather glove.]

  - Place the wet newspaper over the tin to fully seal it.

  - Put a brick or two on top to ensure the seal it good.

 

WAIT until it cools. Then open the tin. You should be surprised how big a vacuum has been generated,

and the tin should have left a clear compression ring in the newspaper.

 

Hopefully the pot should be reduced.

 

If it was a copper-matte "glaze" you may have reduced it so far that there is a layer of metallic copper. You

can carefully reheat this in an oven and watch the colours develop. Unfortunately they fade with time.

 

PS-1

 

A tin may be too weak and implode with the vacuum. I used stainless-steel tea-caddies from a charity shop.

 

You may want to try reducing the degree of reduction. You might try less meths, open sooner, etc.

 

PS-2 I've explained [sic] my limited understanding of copper-matte glazes in post #5 of

http://community.ceramicartsdaily.org/topic/4701-copper-raku-matt-glaze/?hl=%2Bpeter+%2Bmatte&do=findComment&comment=42291

 

PP-3 Dedicated to the memory of Heath Robinson.

https://businessenglishlessonplans.files.wordpress.com/2012/04/self-operating-napkin.png?w=300&h=211

 

 

[bciskepottery]

https://books.google.com/books?id=wEOjXDfq5G8C&pg=PA309&lpg=PA309&dq=achieving+iridescent+glazes&source=bl&ots=sUTuWbS6cR&sig=3eOX2wPEl_mbVH0R6JmoglkE3FI&hl=en&sa=X&ved=0ahUKEwiw-uia2p7RAhUEQSYKHWbACAUQ6AEITzAM#v=onepage&q=achieving%20iridescent%20glazes&f=false

 

https://mattfiske.wordpress.com/2014/04/29/iridescent-glaze-research/

[mousey]

 

 

Last one: https://en.wikipedia.org/wiki/Combustion_chemical_vapor_deposition

 

Combustion chemical vapor deposition (CCVD) is a chemical process by which thin film coatings are deposited onto substrates in the open atmosphere.

Which is what Raku is: depositing a thin film onto a glazed piece. The crystal structure is refracting light back through the thin film; creating the optics of iridescence.

 

 

I have much to learn; I thought 'raku' (in this context) focused on reduction.. I dont see how vapor deposition has any relation to the reduction environment/oxygen depletion..  again my apologies if I'm confusing something, I thought these were two very distinct, very different processes (reduction vs vapor) to obtain luster..

[mousey]

Hi,

 

You might like to try this. You will need:

- A metal canister open at one end a little bigger than your pot.

- Two pieces of chipboard big enough to cover the end of the can.

- A brick or two.

- A newspaper about 1/4 inch thick.

- A bucket of water.

- Rubber bands or string.

- Some charcoal.

- A plastic shot glass or similar.

- Some alcohol (UL meths, US ??)

 

- Place the first piece of chipboard on the ground, with the tin on top. (This insulates the tin a bit.)

- Cover the bottom of the can with charcoal.

- Put the newspaper into the bucket of water to soak.

- When it's getting nearly time to take the pot out of the kiln take the newspaper out of the water.

   Let it drip a bit then wrap it round the chipboard and secure it with the rubber bands or string.

   You should have a nice flat region of paper that will securely seal the lid of the can.

- Fill the shot glass with meths.

- Take the pot out of the kiln, and place it in the tin on top of the charcoal. Keep it away from the sides.

- Rapidly:

  - Pour the meths into the tin. [H&S keep your head well clear, and wear something like a leather glove.]

  - Place the wet newspaper over the tin to fully seal it.

  - Put a brick or two on top to ensure the seal it good.

 

WAIT until it cools. Then open the tin. You should be surprised how big a vacuum has been generated,

and the tin should have left a clear compression ring in the newspaper.

 

Hopefully the pot should be reduced.

 

If it was a copper-matte "glaze" you may have reduced it so far that there is a layer of metallic copper. You

can carefully reheat this in an oven and watch the colours develop. Unfortunately they fade with time.

 

PS-1

 

A tin may be too weak and implode with the vacuum. I used stainless-steel tea-caddies from a charity shop.

 

You may want to try reducing the degree of reduction. You might try less meths, open sooner, etc.

 

PS-2 I've explained [sic] my limited understanding of copper-matte glazes in post #5 of

http://community.ceramicartsdaily.org/topic/4701-copper-raku-matt-glaze/?hl=%2Bpeter+%2Bmatte&do=findComment&comment=42291

 

PP-3 Dedicated to the memory of Heath Robinson.

https://businessenglishlessonplans.files.wordpress.com/2012/04/self-operating-napkin.png?w=300&h=211

 

 

Im oddly terrified of using alcohol in this context..   I've been filling a large dog-bowl with combustibles (rice husks, shredded paper, so on) and placing it on a layer of wet towels, then foisting the hot piece in and once the flames kick up, gently slamming a larger galvanized steel bucket over it for about 15 seconds.. 

 

Whats interesting tho is that although I get really interesting effects from silver nitrate, none of my copper based lusters ever turn that lovely red, and your post makes me wonder if I'm not creating a severe enough vacuum to properly provoke them.

[glazenerd]

 

Combustion (raku) chemical vapor deposition (CCVD) is a chemical process (which includes any and all methods/processes) by which thin film coatings are deposited onto substrates ( your glazed piece) in the open atmosphere.

The solids ( horse hair, rice straw, paper, wood,) leave behind the carbon traces (black color); but they also release vapors ( fuming ) particles of potassium, sodium, metal ions: particulates.

 

People think of "reduction" as a non-oxygen atmosphere (partly true), but in reality it is creating a Redox environment. In glaze chemistry: we deal with molecules (two or more atoms held together by a crystal lattice) think(molarity) (molecular weight).

 

Redox (reduction/oxidation) is the gain or loss of electrons ( has nothing to do with kiln environments), other than a CO environment accelerates the chemical reaction of Redox. When a glaze is reduced in a firing; that really means the number of oxygen molecules have been reduced: which creates a change in color. That color change is the result in a change in the cleavage (angle the light is reflected/defracted, and or absorbed).

 

Think of the prism shown above as your glaze (without color). The amount of oxygen molecules determine the angle the light is deflected. Molecules have a crystal lattice: Silica is SiO2  one silicone molecule held together by two oxygen molecules. (actually it is a tetrahydra SiO4, but keep it simple) When you melt the glaze; the SiO2 becomes (thermal) dissociated. The Si and the O2 are floating around in your molten glaze soup, along with the metal ions. As it cools, these atoms begin to form a chemical bond (covalent) in an amorphous (fancy word for glass) structure.

 

• Oxidation is the loss of electrons or an increase in oxidation state by a molecule, atom, or ion.
• Reduction is the gain of electrons or a decrease in oxidation state by a molecule, atom, or ion.

When you oxidize copper, it is a variety of green colors. ( because you have increased the oxygen molecules)

When you reduce copper, you have decreased the oxygen molecules......explanation coming.

 

Reduction firing "reduces the oxygen molecules:, which in turn changes the crystal lattice, which in turn changes the angle at which the light is reflected back through the glaze: which in turn changes the "appearance" of the color to the human eye.  More oxygen molecules make it appear green to us, and less oxygen molecules makes it appear red to us.  The change in lattice, changed the way our eye perceives color.  The prism above explains that: each color is represented by a slight rotation of the angle of reflection.  Reducing or oxidizing a glaze changes the crystal lattice, which in turn changes how it refracts light back through the glaze.

 

I kept it as simple as I could, omitted much, and over-simplified the rest.

 

Nerd

[mousey]

my mind is absolutely spinning, taking the time out to explain this to me (simplified or no) is remarkably kind and generous, i cant think you enough.  

[bciskepottery]

 

 

Last one: https://en.wikipedia.org/wiki/Combustion_chemical_vapor_deposition

Combustion chemical vapor deposition (CCVD) is a chemical process by which thin film coatings are deposited onto substrates in the open atmosphere.

Which is what Raku is: depositing a thin film onto a glazed piece. The crystal structure is refracting light back through the thin film; creating the optics of iridescence.

 

 

I have much to learn; I thought 'raku' (in this context) focused on reduction.. I dont see how vapor deposition has any relation to the reduction environment/oxygen depletion..  again my apologies if I'm confusing something, I thought these were two very distinct, very different processes (reduction vs vapor) to obtain luster..

 

I am rather confused . . . the process of CCVD described in Wikipedia -- where the vapor is part of the flame -- in unlike any raku process I've heard of, either traditional Japanese or American. My understanding is in American you can add combustibles to the reduction container (e.g., miracle gro) to get flashing etc., or you can do it with saggar firings, but that is not the same as described in the CCVD excerpt.

[Marcia Selsor]

Yes, Nerd.

Thank you for that very scientific explanation. with diagrams even. Well done.

Marcia

[glazenerd]

Bruce:

 

Raku or Saggar firings are sort of a hybrid process: they actually are more closely related to an "annealing" process. In which we are heating the piece until glowing; and then treat it: but then again it does not apply. CCVD is the closet relative process than describes the raku process. There are deviations of the CCVD, but they include injecting gases, using pressure chambers, or utilizing electrolysis.  CCVD heats the substrate and injects chemicals at the same time: powder coating is an example. We heat the piece first, then apply chemicals: the only real difference. The end result is still a thin layer is deposited. CCVD is the only chemical process that most closely resembles what raku is doing. There could be another, but I have not found it yet: and I am looking.

 

Nerd

 

Mousey, I answered because you asked politely. Some demand answers, and those I tend to walk away from.

[glazenerd]

Lesson 2    ( I came this far, why stop now.

 

 

The above is showing a Redox reaction: the sodium is losing a molecule and the fluorine is gaining one. (More specific terms, but leave it at molecules) When elements share molecules: it is called a covalent bond. When molecules are held together by electrostatic charges: it is called an ionic bond. Crystals are an ionic bond; see my avatar.  The sodium is losing one, so it is called the reductant (reduced). The fluorine is gaining one, so it is called the oxidant (oxidized). Most all glazes used are covalent bonds: they are chemically bond together: sharing molecules.

 

 

Every element we use in glaze has a crystal lattice (atomic structure). This illustration of sodium shows NA (sodium atoms in purple), held together by oxygen atoms in green: combined they form a molecule. So when you fire a glaze to peak; you are firing to a temperature that causes the lattice to break apart ( thermal disassociation): which we call a melt. The reason we call lithium, sodium, potassium, and calcium fluxes; is because they are the easiest to break apart (melt). When they break apart, then they begin to cause the other elements ( transitional metals: copper, zinc, cobalt, etc) to melt; and when they melt, combined they begin to melt the silica.  Look at the periodic table: the ones on the left are the easiest to disassociate, and the ones on the right are the hardest to break apart.

 

 

So after everything is melted at peak temperatures: all these atoms are floating around in glaze soup (eutetic). The illustration above shows two of many ways the atoms begin to reform as the glaze cools. The above example is an ionic bond: because they molecules are arranging in a repeated pattern (HCP or PCP) HCP is hexagonal closed pack: or what we call in the glaze world: crystals. When they are more random, they are called covalent bonds. As they reform, a crystal lattice is formed: and that is what determines how the light is refracted back through the glaze: determines the color we see.

 

 

 

 

The silica we use does not really have color per se, but it is the most responsible for how the color is refracted back. Notice the bottom of the crystal looks matte, because the light is diffused. The center looks clear (gloss) and towards the top the light is bent: showing color.

Part 2 below

Nerd

[glazenerd]

It creates planes, also called cleavage, and also called angle of deflection, but mainly called refractive index. In oxidation, there are more of these silica molecules: which reflect light on one plane, but when you reduce them (reduction fire), there are less of them: which refract light on a different plane. (different color).

So the large chunk of quartz (silica) bends light so it looks like this:

Metals absorb light; so when we add metal oxides or pigments: how light is refracted changes again.

 

Silica is crystalline: perfectly repeating lattice pattern. Feldspars are poly morphs (crystalline): and when they form bonds upon cooling: you have an amorphous lattice (glaze). When you purposely formulate a glaze recipe to create an ionic bond, instead of a covalent bond: you get crystals

 

Nerd

[Guest JBaymore]

Nice work, Nerd.  You did miss one point in the above that relates to American Raku.  And the first "in your face" effect that tends to "grab" everyone's attention.  Copper luster.

 

If you reduce oxides of copper enough,...... what you are left with is...... copper metal.  Copper luster is a VERY thin layer of copper metal particles floating at or near the surface of the transparent (or even still green or red) glaze below it.  To achieve this, you need to hit the molten glaze very hot, and with very strong reducing conditions.  And hold that reduction situation until the copper surface is not able to re-oxidize when oxygen is again present.

 

Note that copper lusters tend to tarnish and gain a patina over time when exposed to air (with its oxygen).... just like any copper object.  Some American raku artists use sealants (acrylics typically) to stop this process form happening over time.

 

best,

 

.......john

[glazenerd]

Thank you professor: the addition is well placed.  I was trying hard not to overwhelm with the information dump.

 

Hey!!! You called me Nerd.. TY.

 

Tom..LOL

 

Now, with all this knowledge; I will start experimenting with bending the refraction angles to create matte: without changing the Si/AL levels. Should be fun: think I will call it the Boris Karloff effect.

[Dick White]

I am going to jump in here and disagree with Nerd on a few things, not because I am smarter than him (no way, ever, he brings so much to the table and I am constantly learning from his generous shared wisdom) but because I have messed around with both standard reduction firing in gas-fueled kilns and several forms of Western-style raku.

 

Up above in post #13, Nerd said, "Redox (reduction/oxidation) is the gain or loss of electrons ( has nothing to do with kiln environments), other than a Co2 environment accelerates the chemical reaction of Redox. When a glaze is reduced in a firing; that really means the number of oxygen molecules have been reduced: which creates a change in color. That color change is the result in a change in the cleavage (angle the light is reflected/defracted, and or absorbed)." A few nits based on my experience.

 

Yes, redox is gain or loss of oxygen electrons in the molecular matrix. No, for us it has everything to do with kiln environments, and the primary accelerant of reduction in a kiln or raku post-firing reduction chamber is not CO2, but is CO. The raw materials we use (e.g., flux and glass-forming minerals, colorant oxides) exist at ambient temperatures in a variety of mixed, but stable, molecular states, i.e., they are neither trying to give up excess oxygen atoms nor trying to find stray oxygen atoms to complete their atomic structure. Metal colorants (except perhaps red copper oxide) are stable oxides, dioxides, or carbonates, depending on their underlying valence. Oxygen atoms are neither entering or leaving at ambient temperatures. At the extremely high temperatures when the glaze melts, the situation changes. Various molecular bonds of the raw materials disassociate and new molecular bonds form in the molten glaze to create the glassy matrix. But at all times, the various oxide molecules seek stability with the correct number of oxygen atoms. Unneeded oxygen is simply released into the surrounding atmosphere, often in the form of CO2 released as the carbonate aspect decomposes.

 

We sometimes incorrectly refer to an electric kiln firing as an oxidation atmosphere, but technically it is a neutral atmosphere, the same as the natural air we breathe. I suppose one could construct a kiln in which a tank of oxygen constantly pumps more oxygen into the kiln as it fires, but for our purposes, such would produce no useful change from what we can already get just firing in plain air, so why bother with the added complication. The ceramic oxides already have what they need to remain stable, and the metallic oxide colorants form their usual colors - cobalt oxide blue, copper oxide green, etc. Reduction in a ceramic kiln is caused by introduction of a completely different atmosphere. In a gas (or other fuel) kiln, the air-to-fuel ratio is manipulated to produce incomplete combustion of the fuel, resulting in the creation of unstable carbon monoxide as a combustion by-product rather than the stable carbon dioxide of a clean burn. The resultant effect is that the unstable CO seeks to stabilize itself by scavenging an oxygen atom from wherever it can find one in the melting glaze. An atomic tug-of-war starts as the the CO grabs at oxygen atoms. The basic ceramic oxides (fluxes and glass) tend to have a stronger hold on their oxygen atoms than the metal colorants, so the CO grabs its needed extra oxygen from the metals and color changes occur. Red iron oxide changes to yellow, black, blue and green depending on the degree of reduction and other oxides present in the glaze; green copper oxide changes to elemental copper red; etc. There is no deposition of new material onto the surface of the glaze, i.e., CCVD is not involved, other than perhaps contamination of the molten glaze by carbon soot if the reduction is too aggressive (and sometimes this is desirable, as in carbon-trapping shinos).

 

In the raku scenario, we typically have less control over the atmosphere in the kiln as it heats, though one can modify the atmosphere somewhat by moving the burner a bit to change the amount of secondary air coming in or sliding a brick across the flue exit. The reduction, and consequent color change in glazes that are susceptible to that, occurs in the post-firing bin of combustibles. The red-hot pot is moved from the kiln to the pile of flaming trash as fast as possible so that the glaze remains molten, and then the lid is put on as the flames lick around it. Because the lid is (or should be) relatively tight, the flaming trash quickly consumes the available oxygen in the bin and soon begins to generate the CO characteristic of incomplete combustion.  As this all happens very quickly and the glaze surface is hopefully still molten, very localized reduction occurs where the flames are licking over the pot and copper greens flash to red, blue, and purple. Again, no CCVD is occurring, nothing new is being deposited on the glaze surface. However, the blues and purples are not from the usual elemental copper (red) but rather because the reduction to elemental copper is only a few atoms deep. The reflectance/refraction is different when the elemental copper is that thin. This is also the reason why so much of the beauty of copper raku glazes is fugitive after a few years of exposure to air - it naturally reoxidizes back to blah. Some raku artists apply sealants to delay this, but long-term preservation is difficult.

 

I suppose you could describe the carbon-blacking of any exposed bare clay surface while in the post-fire reduction bin as a form of CCVD, in that the hot ceramic has an affinity for the carbon soot in the smoke of the dying flames of the combustibles, but the effect is nothing like the deposition of thin film lusters of typical CCVD. But before I completely dismiss CCVD, there is a raku technique involving fuming with stannous chloride. These pieces are multi-fired, sometimes with precious metal luster glazes added, and the stannous chloride is poured onto hot bricks placed in the final firing, which then vaporizes and fumes the substrate glaze. But this process is quite involved, there are more technical hazards than in typical raku firing, though this may be what the original question was about - the rabbit hole of lusters...

 

Cheers and happy new year to all

dw

[Min]

"I am going to jump in here and disagree with Nerd on a few things, not because I am smarter than him (no way, ever, he brings so much to the table and I am constantly learning from his generous shared wisdom) but because I have messed around with both standard reduction firing in gas-fueled kilns and several forms of Western-style raku.â€

 

@dw, Your experience speaks volumes. I also appreciate the maverick way of thinking Nerd brings to this forum. I don’t think anyone here has any problems with a peer review. 

[glazenerd]

DW:

 

I kept it as simple as I could, omitted much, and over-simplified the rest.

I fully realize that I left out some fine points, I did so intentionally. I appreciate the review and absolutely defend your right to disagree..

 

 

Various molecular bonds of the raw materials disassociate and new molecular bonds form in the molten glaze to create the glassy matrix.    We both stated this... Agreed

 

 

But at all times, the various oxide molecules seek stability with the correct number of oxygen atoms.   ....in theory yes, but there are overriding chemistry laws:  Reduction potentials, rate laws, valence bonds, band gaps, and more.  Crystalline is ZNO and SiO2 when it goes in kiln, and comes out Zno2 SiO4: formed by ionic bonding. ZNO has a band gap of 0.283 and Sio2 is 0.033 : the band gap of ZNO is higher than silica: therefore reduces the silica.

 

 

 

Unneeded oxygen is simply released into the surrounding atmosphere   ... this would be thermal decomposition, not disassociation.  In the U of I study, feldspars (sodium and potassium) thermally decomposed at 2190F, and were no longer visible on x-ray defraction. So, on this wise applicable because both fluxes convert from solids to gas in their phase changes.

 

 

The basic ceramic oxides (fluxes and glass) tend to have a stronger hold on their oxygen atoms than the metal colorants, .. we are now talking reduction potentials. Again referring to the periodic table: moving left to right (in part) defines how easily an element can be disassociated.

 

Compound                         Periodic Name                  Isoelectric Value                              Melting Point

Tungsten Oxide                                WO(3)                                 0.2-0.5

Antimony Oxide               Sb(2)O(5)                                            0.4-1.9

Vandium Oxide                 V(2)O(5)                                              1.2-3

Silica Dioxide                      SiO(2)                                               1.7-3.5                                  3025F

Tin Oxide                             SnO(2)                                               4.5-5.7

Mangenese (Di)Oxide   MnO(2)                                                   4.1-5.0

Titanium Dioxide              TiO(2)                                                    3.9-8.2*

Rutile                                    TiO(2)*                                                 3.9-8.3

Iron Oxide                           Fe(3)O(4)**                                       6.5-6.8                                  2850F

Iron Oxide (alpha)           Fe(2)O(3)***                                    8.4-8.6                                  2850F

Chromium Oxide              Cr(2)O(3)                                             6.2-8.1

Alumina Oxide                  Al(2)O(3)                                             8.1-9

Copper Oxide                    CuO                                                       9.5-9.6

Zinc Oxide                           ZnO                                                        8.7-10.3                                3587F

 

My personal reduction potential/ isoelectric point chart.. feel free to cut and paste it  ...from UC-Berkley

 

 

 

But this process is quite involved, there are more technical hazards than in typical raku firing, though this may be what the original question was about - the rabbit hole of lusters.. correct. or that was the way I took it anyway. 

 

Nerd

 

Note: I do not view myself as being smarter than anyone else on the forum: just an area I have studied for a long time.

[glazenerd]

This might interest some.

 

http://web.mit.edu/2.813/www/readings/Ellingham_diagrams.pdf