Tag: recipe

Shizuku Yuteki Variation

Here’s another Oil Spot I put up on Glazy.org.

You might notice that this one has a significant addition of Cobalt, half and half Custer (Potash) and F4 (Soda) Feldspars, calcined talc, and 2% Manganese. Typical that I changed too many things to give a really useful side-by-side comparison. But I suppose when I’m coming up with new variations, that’s always been my style.

2 Shizuku Yuteki Variation
Left: Underfired and immature. Cone 9/10. Right: Super long and hot cone 12 firing, with plenty of peak soak time.

Some observations on this one:

Cobalt goes a long way and pretty dramatically alters an oilspot. With a  .25%-.5% addition you get a nice shift from brown and russet glaze matrix to a darker solid black glass. Beyond 1% you can get some really nice silvery qualities to the spots. The drawback is that the more you add, the more refractory the glaze tends to get – and the longer it takes for the glazes to heal.

Continue reading “Shizuku Yuteki Variation”

Basalt as Colorant in Celadon Glazes

Basalt as Colorant in 2 Base Recipes.
Basalt as Colorant in 2 Base Recipes.

More local Basalt. Here used as colorant in high fire celadon glazes. On the top left, the raw material which was collected from various places throughout Idaho and Utah (and all mixed together), bottom left the homogenous, calcined, milled, sieved, and dried material ready for glaze.

In this set the basalt is supplying the iron necessary for that timeless celadon blue. Its also bringing significant additions of magnesium and calcium to the recipe. The % of basalt here ranges from 0 to 10% in 2.5% steps – applied to a dark stoneware and porcelain tiles.

This series were fired in a very fast and simple cone 10 reduction firing with a very basic reduction cool. 6 hours start to finish, in a small fiber test kiln — Heavy body redux for 30 min @ ^012-^08, then light redux to ^6, then a medium redux to ^10. At soft cone 11 I crash cooled a few hundred degrees, turned the air and gas down, dampered in, and put the kiln into about a -4°/minute cool, periodically opening the door to quickly crash cool -30 or -50 degrees until 1400, then shutting everything off. In some cases reduction cooling will effect the color and quality of the glazes significantly, but here it only effected the stoneware – keeping the iron oxide on the surface in its black reduced form. A good reduction firing will yield these glaze colors with no special effort cooling – here the RC was strictly for a darker stoneware color.

The Recipes

Fiske’s Tichane Chun
Custer Feldspar 48
Silica 31
Calcium Carb. 20
Bone Ash 1
(Iron Oxide 1.5)
— A range .5 to 3% Iron Oxide gives a similar spectrum of blue as the basalt does here – different flavors of Iron bearing materials yield different flavors of glaze, obviously. I’ve tried probably more than 50 kinds of iron over the years – try what you have and figure out what flavor you like best!

Fiske's Tichane Chun with 1.5% Red Iron Oxide. Fired to C10 in Reduction.
Fiske’s Tichane Chun with 1.5% Red Iron Oxide. Fired to C10 in Reduction.

Fiske’s (Pinnell Clear) PC Celadon
Custer Feldspar 25
Grolleg Kaolin 20
Calcium Carb. 20
Silica 35
(Spanish Iron Oxide .85)

Fiske's PC Celadon with a range of 0%-2.55 Red Iron Oxide. Fired in C10 Reduction.
Fiske’s PC Celadon with a range of 0%-2.55 Red Iron Oxide. Fired in C10 Reduction.

 

Basalt

A Basalt Quarry near Paul, Idaho.
A Basalt Quarry near Paul, Idaho.

For a very long time now I’ve wanted to utilize some volcanic rock as glaze. In much of my research here at Utah State I’ve been looking at iridescent phenomena, both in glazes and in the natural world. It was quite fortuitous, then, when geology grad Doug Jones asked me to accompany him on an excursion just over the border into Idaho to look for Xenoliths, which at this site are very deep mantle rocks that have been blasted quickly to the surface in younger volcanic flows.

While we were poking around looking for Xenoliths, I started picking up some rather remarkable chunks of iridescent vesicular basalt. Vesicular basalt is characterized by it’s frothy, bubbly matrix… if you don’t know what I’m talking about, think red lava rock. It’s one in the same. Here’s an example:

Vesicular Basalt
Vesicular Basalt

After picking up a good pile of this stuff, we went on to find about 40 Xenoliths, as well as some other interesting stuff.

 

Basalt with Quartz Clusters
Basalt with Quartz Clusters

Once I got back to the studio it was time to figure out if this stuff was even viable. My standard go to for this is to break off a small chunk, put it in a dish, and fire away.

wpid-20150222_135231.jpg

After putting theses samples in a cone 10 reduction kiln and a cone 10 oxidation kiln, it became quite evident that I had something useful.

wpid-wp-1426884851127.jpg
Fired Samples

After deciding that this was a good road to go down, the hard work of crushing and processing this stuff began. I started by breaking the boulders down into gravel sized pieces. These then went into out ball mill. I could have shaved down the time it took to mill this stuff by using intermediate crushing equipment (an impact mill, or hammer mill) but I found it easier in the long run to load up our ball mill and run for about 24 hours, sieve out the useful material, add in more course material, and repeat. By the end of 4 days I’d run all the material through and was left with tumbled lava rock:

With my material milled down fine enough to pass easily through a 100mesh sieve, I then let it sit for a few days, pouring off the water each morning, until gradually the material became thicker and started to resemble a glaze. Because it was ball milled, the particles were quite small, and suspend really well.  The next step was to take this glaze material and see what happens in the kiln. I was quite pleased:

Basalt Material fired to 1250C in Reduction
Basalt Material fired to 1250C in Reduction
100 Basalt Glaze Material on Porcelain, fired to 1265C in Oxidation
100 Basalt Glaze Material on Porcelain, fired to 1265C in Oxidation

 

Now that I knew I was dealing with a viable glaze, I couldn’t wait to get this stuff in the kiln and firing it in a weird, experimental reduction cool cycle.  Last year I discovered some really incredible surfaces by cooling a kiln in reduction, and holding at certain temperatures. In this case, the geologists have told me that basalt solidifies at about 980C, so I’ve been crash cooling the kiln to around this temperature, holding in a reduced environment, and letting the metallic compounds crystallize in reduction. My speculation is that I can somewhat re-create the conditions in which iridescent phenomena occur. Lo and Behold:

Iridescent basalt Sample left, Iridescent Glaze right.
Iridescent basalt Sample left, Iridescent Glaze right.

This result is remarkably similar to effects you can achieve in Raku, or Lustre firing… but it’s a different phenomena, and relies on totally different elements; namely, Iron. Whereas raku usually derives rainbow iridescence from Copper and Cobalt, and lustres from Silver, and Bismuth, these colors are coming from Iron with trace amounts (less than .5% Manganese and Titanium). It’s very interesting on the ceramics side, and the geologists are quite interested too, because the phenomena is not wholly understood.  One of the perks of being a graduate student with STEM funding is that I have access to fancy analytical equipment.  This analysis has allowed me to build a material profile in Insight Glaze Software.

 

Insight Profile for this Basalt Material
Insight Profile for this Basalt Material

To that end, my future plans with this research will involve more experimentation with the firing process. In fact, I’m currently working on a piece that will exhibit between 10 and 20 wall hanging tiles that all feature the exact same clay and glaze with different firing schedules.  At the same time, I’ll also continue to tweak this material by adding other oxides to end up with brand new flavors of glaze.

 

80% Basalt Glaze Material + 20% Porcelain Clay Body
80% Basalt Glaze Material + 20% Porcelain Clay Body

Fiske ^6 Celadon

Fiske Celadon Blue
Gin&Tonic Cocktail Glasses

 

I get asked a lot about this recipe, and for good reason. It’s pretty indistinguishable from the best cone 10 recipes out there. For those purists out there, I’m referring to Pinnell Clear, Deller Chun, Cushing’s LungChun, and any number of Robert Tichane’s recipes from his book Celadon Blues. In any event, I often point people to an older post, but in the years since I mistakenly transcribed a recipe wrong and happened on the winning formula, I’ve learned quite a bit working with this glaze; Things like it working reasonably well in soda and atmospheric kilns, looking very nice from a range of cone 5 to cone 12, and readily taking most mason stains.

The Recipe

Fiske 6/10 Clear Base:

F4 (Or MinSpar) Feldspar 34.9

Whiting 12.8

Zinc Oxide 11

OM4 Ball Clay 13.8

Silica 27.5

(Pictured: Add 1.75% Robin’s Egg Blue Mason Stain)

 

Notes on materials, mixing, and application:

Feldspar:Since F4 is no longer widely available, Minspar 200 will work. Custer works as well, but the bubble matrix that really gives this glaze it’s character is different with custer, g200 (now g200hp), or nepheline syenite. Experiment first, because milage may vary. Of the ingredients, this is probably the 3rd most important.

Fluxes: Whiting and Zinc. This glaze is not kind to kiln elements. (See my post on zinc for clarification!) It’s my opinion that the relatively high % of zinc is caustic to electric kiln elements. If you must, ventilate the kiln, but expect a short life on the elements. Sometime in the near future I’ll be eliminating zinc and trying to use a frit to solve this, but until then I can only recommend firing in a gas kiln. It’s the cost of firing.

Clay: Probably the most important element of this recipe. When I was testing for cone 6 glazes, I made a mistake transcribing to a batch recipe. The result was that I had doubled the clay. After the firing I went back over the notes and realized why the glaze looked the way it did. One of the side effects of the higher clay content is that application is sometimes difficult. The higher % of clay makes thick applications crawl. To get around this I calcine 10 of the 15%.

Thus, my recipe looks like this:

Fiske 6/10 Clear Base: Minspar200 Feldspar 38, Whiting 14, Zinc Oxide 12, Calcined OM4 Ball Clay 10, OM4 Ball Clay 5, Silica 30. [H20 60%]

Silica: I use 200mesh sil-co-sil. I’ve tried 325 mesh, but it didn’t look right.

Colors/Mason Stains: I use Robin’s Egg Blue, Bermuda Green, and Canary yellow. Most colors I’ve tested, and usually 1.5-3% is pretty nice, but some take as much as 5-10%. I haven’t had much luck with purples, Pinks, and oranges, (they don’t play nice with the zinc) but honestly I’m largely done tweaking this one and haven’t tried in earnest to figure out those other colors.   Metallic oxides will also work, cobalt at like .3% for a not overpowered blue color.

Application: Can’t stress this enough. It’s gotta be thick. I tell people to glaze “Thicker than you think thick is, and then just a little thicker.” I’ve taken to adding just a touch of deflocculent to the glaze batch so that it needs less water to become liquid. I then add a bit of Epsom salt to thicken the batch up. Again, this is to taste. Dipping is absolutely the way to go with this one, but I’ve gotten accustomed to spraying it. Usually takes about 15 minutes to spray glaze something appropriately.

Firing: As I mentioned earlier, it’s got a pretty wide range. It will be fully melted, albeit slightly pin holed at cone 5. Ideally, I like to go to a perfect 6, but taking it to 7 or programming a hold in the schedule makes for some nice movement that suits carving and texture very well. Most of my work is completely smooth, so I prefer it to stay thick and not run down too much. It takes some getting used to, but when you do, it behaves very predictably. It can also go into reduction, but the colors change quite a bit. Less change with Bermuda Green, but quite a bit with the Robin Egg Blue. Its been fired every which way, and needs to be tested before full comittment.

Rainbow Iridescent OilSpot Glazes

Hello Again! It’s been quite some time since my last post. Gotta thank those of you who have contacted me with interest and suggestions! With so many summer projects and school stuff, it’s been very difficult to put my full efforts into any one thing… but life is what happens while you’re making plans.  Anyways, enough with the excuses.

Over the summer I had the time and energy to figure out an acceptable firing schedule in our new Blaauw kilns.  For as much as I love their sleek and sexy design, computer controllers, and top of the line hardware… you can’t look in the damn things while they’re firing. This poses several challenges for control freak oil spotters. Usually, the idea is to firein complete and total oxidation, going slowly through cone 7,8,and 9 to allow thermally reducing iron to bubble up through the glaze and cause the surface to crater or foam. By carefully monitoring the situation inside the kiln, and by pulling out glazed pull rings, the firer can increase the temperature slowly and fire until the glazes have significantly ‘healed over’. This isn’t really an option, so as a result a much more empirical approach was needed to find a good fit.

After 5 firings, I settled on a more or less acceptable firing schedule (the way this programming works is that the kiln starts at 0, take 1:30 to get to 200C, then 2:30 to get to 700C, etc). In Celcius;

time_temp 00:00 5

time_temp 01:30 200

time_temp 02:30 700

time_temp 03:00 1115

time_temp 02:00 1190

time_temp 02:30 1230

time_temp 02:30 1253

cooling

time_temp 02:00 1000

time_temp 02:00 500

time_temp 02:00 300

time_temp 02:00 50

time_temp 04:00 50

 

Once that was established, I began with some of my favorite tiles from my initial 2 rounds of oilspot base glaze recipes. My favorites:

 

NoCo OS:  (NC)

Dolomite 4.4

Whiting 4.4

K200 Feldspar 57.3

EPK 9.7

200m Silica 24.2

Spanish FeOx  10

 

Candace Black:  (CB)

Dolomite          5

Whiting           5

K200 Feldspar 60

EPK                       5

200m Silica  20

Spanish FeOx  8

Cobalt Carb       5

 

Loganspot: (LS)

Local Black Dolomite 10

K200        65

EPK          5

Silica     20

Cobalt      5

Red Iron  8

 

Fake Mashiko: (FM)

K200  37.6

Silica  9

Redart  8

Calcined Redart 35

Wollastonite   5.7

Talc    4.3

Bone Ash  .5

Red Iron   4

 

With these base glazes I began mixing, blending, and layering, and combining glazes with dipped, poured, and sprayed application.  On a whim I decided to experiment with some of my manganese saturate glazes, and that’s when things started to get really interesting. There is admittedly one glaze in particular that I’m not sharing, but with a little diligence and some wet blending, a seriously motivated glaze experimenter can discover this glaze by  looking at my old posting on my OSII series. Blend them all in 50/50 proportions and you’ll get the elusive but beautiful  GF glaze. Hell, it might even be on my blog somewhere. That’s all I’m saying for now – I’d hate to rob anyone of the learning experience… Hah! =)

 

Recently I was contacted by the British potter Allen Richards who has done some pretty extensive research into lustrous gold glazes. He suggested that I try small additions of Vanadium Pentoxide. These glazes feature 2 amended manganese saturate glazes in combinations with the usual oilspot suspects.

 

 

 

 

Here are some videos of some of my latest results. None of these particular tiles have Vanadium pentoxide.  As time goes by I’ll try to annotate the combinations MS corresponds to Manganese Saturate.

 

 

Iridescent Hare’s Fur Tenmoku

New Pots, New Recipes

#6, #5, #4 Manganese Saturate Crystalline Glazes
#6, #5, #4
Manganese Saturate Crystalline Glazes

#4 Recipe & Schedule

Fisker Bronze
Custer Feldspar.............     57.000  
  Alberta Slip................    7.000  
  Sil-co-sil..................    2.500  
  F-4 Feldspar................    1.500  
  Calcium Carbonate...........    0.500  
  Dolomite....................    0.500  
  OM #4 Ball Clay.............    2.000  
  MnO.........................   23.000  
  Copper Carbonate............    5.500  
  Iron Oxide Red..............    0.500  
                              =========
                                100.000

  Oxide   Formula  Analysis    Molar%
  CaO      0.043*   1.129%w   1.376%m
  MgO      0.019*   0.348%w   0.590%m
  K2O      0.143*   6.237%w   4.528%m
  Na2O     0.071*   2.052%w   2.264%m
  P2O5     0.000*   0.007%w   0.003%m
  TiO2     0.001    0.055%w   0.047%m
  Al2O3    0.252   11.952%w   8.013%m
  SiO2     1.783   49.756%w  56.619%m
  CuO      0.099    3.670%w   3.156%m
  Fe2O3    0.013    0.955%w   0.408%m
  MnO      0.724*  23.839%w  22.995%m

                        Cost:   0.273
              Calculated LOI:   3.521
                 Imposed LOI:        
                       Si:Al:   7.066
                      SiB:Al:   7.066
           Thermal Expansion:   6.848

Fired in Blaauw Reduction Schedule (in Celcius):

time_temp 00:00 5
time_temp 00:54 140
time_temp 01:12 260
time_temp 01:10 550
time_temp 00:30 600
time_temp 01:12 900
oxidation 83
time_temp 00:45 900
oxidation 93
time_temp 03:06 1210
oxidation 98
time_temp 01:24 1270
cooling
time_temp 02:15 1000
time_temp 01:00 900
time_temp 02:00 500
time_temp 01:00 300
time_temp 02:30 50
time_temp 04:00 50

#5 Recipe & Schedule

  Nepheline Syenite...........   65.500  
  MnO.........................   22.000  
  Silica......................   12.500  
                              =========
                                100.000

  Oxide   Formula  Analysis    Molar%
  CaO      0.018*   0.465%w   0.559%m
  MgO      0.004*   0.067%w   0.112%m
  K2O      0.071*   3.036%w   2.175%m
  Na2O     0.228*   6.461%w   7.033%m
  Al2O3    0.330   15.361%w  10.163%m
  SiO2     1.913   52.523%w  58.975%m
  Fe2O3    0.001    0.072%w   0.030%m
  MnO      0.680*  22.014%w  20.953%m

                        Cost:   0.312
              Calculated LOI:   0.065
                 Imposed LOI:        
                       Si:Al:   5.803
                      SiB:Al:   5.803
           Thermal Expansion:   7.492
              Formula Weight: 218.874

Strike Reduction Hold Firing Schedule 
in Small Test Gas Kiln in F

3:30 -> 1500F (^012)
Body Reduction
1:00 -> 1700F (^04)
Adjust to Moderate reduction, fast climb
3:30 -> 2300F (^9 flat, ^10 down)
Crash Cool
0:15 -> 1840F
Cut secondary air, minimize primary air, damp in, gas low to strong reduction and stalled holding temp
3:00 -> 1840F (Hold)
Off, Natural Cool
6:00 -> 300F

#6 Recipe & Firing Schedule

  Custer Feldspar.............   69.000  
  OM #4 Ball Clay.............    1.500  
  MnO.........................   27.500  
  Granular Manganese..........    2.000  
                              =========
                                100.000

  Oxide   Formula  Analysis    Molar%
  CaO      0.007*   0.210%w   0.256%m
  MgO      0.000*   0.006%w   0.010%m
  K2O      0.140*   6.961%w   5.057%m
  Na2O     0.064*   2.091%w   2.308%m
  TiO2     0.000    0.018%w   0.016%m
  Al2O3    0.226   12.223%w   8.201%m
  SiO2     1.533   48.751%w  55.512%m
  Fe2O3    0.001    0.123%w   0.053%m
  MnO      0.789*  29.617%w  28.588%m

                        Cost:   0.297
              Calculated LOI:        
                 Imposed LOI:        
                       Si:Al:   6.769
                      SiB:Al:   6.769
           Thermal Expansion:   7.120

Fired in Blaauw Reduction Schedule (in Celcius):

time_temp 00:00 5
time_temp 00:54 140
time_temp 01:12 260
time_temp 01:10 550
time_temp 00:30 600
time_temp 01:12 900
oxidation 83
time_temp 00:45 900
oxidation 93
time_temp 03:06 1210
oxidation 98
time_temp 01:24 1270
cooling
time_temp 02:15 1000
time_temp 01:00 900
time_temp 02:00 500
time_temp 01:00 300
time_temp 02:30 50
time_temp 04:00 50

Iridescent Glaze Research

Download the full PDF of my research Paper:

Iridescent Glazes

Download the full Powerpoint of my research Presentation:

Iridescent and Manganese Crystalline Glazes

 

 

 

Text From Paper:

 

Matt Fiske

Technology of Ceramics, Glaze Calc

April 24, 2014

Iridescent and Manganese Crystalline Glazes

Manganese crystalline glazes (high alkali, silica, and alumina) are usually created by saturating a feldspathic glaze with between 15-60% manganese dioxide. During the cooling cycle, manganese precipitates out of the molten glaze and crystallizes on the surface, producing lustrous, satiny surfaces.

UNDERSTATEMENT: Manganese Dioxide is extremely hazardous to your health!!!

 Breathing in Manganese dust when mixing these glazes or breathing the off-gassing vapor when firing WILL GIVE YOU PARKINSONS-LIKE SYMPTOMS BEFORE ULTIMATELY KILLING YOU, PAINFULLY. HEAVY GLOVES, DUST MASKS, AND VENTILLATION ARE CRITICAL.

 

Historical Information

            There is a long history of lustrous, metallic glazes. The first examples are thought to be from the early ninth century in an around what is modern day Iraq. Archeological evidence suggests that early examples originated from Mesopotamia in Fustat, which was then the capitol or Egypt. The oldest surviving examples were often multi-colored stains and iridescent sheens derived from copper and silver compounds. These compounds were usually manufactured by dissolving coins into acids and then mixing the resulting solution with earthenware clay. This mixture was then calcined and then finely ground. The resulting pigment was then mixed with a carrier (usually lavender oil) and applied to lead or tin glazed pots and re-fired to dull red heat. The pots were then held in an extremely smoky reduction environment at various temperatures and lengths of time, which resulted in surfaces ranging from olive-green, brown, amber, orange, yellow, crimson, and a very dark red which was sometimes so dark as to look almost black.[1]

Although the history and development of reduced-pigment lusters is long and storied, it was a more or less consistent sequence. It isn’t until the 19th century that one starts to find examples of resinate lusters. This resulted in the development of materials almost identical to modern ‘liquid gold’ and ‘platinum’ lusters. In Europe in the 1870s a revival in the technology and development of luster glazes saw a further refinement of reduced glaze lusters, most notably in the studios of William De Morgan, Massier, Kähler, and Zsolnay. This notable shift was the result of the use of higher firing clays, which French ceramicist Louis Franchet believed could offer the complete range of earlier pigment-lusters, but without a lot of the trouble.[2] Aside from the obvious temperature differences, the main difference between pigment and reduced glazes is that glaze lusters are generally less subtle, less mellow, and offers a wider, more brilliant range of color.

Abstract

I began research on this project in an attempt to find a brilliant, iridescent glaze similar to Zsolnay’s famous Eosin glaze, which has a very obvious bright reflective rainbow iridescent quality. Initial research suggested that Zsolnay’s effects were the result of the thin application of copper, silver or bismuth to a pre-fired glaze – firing to fusion point, and then reducing the kiln atmosphere during the cooling cycle. This method is documented extensively in Greg Daly’s book Lustre. Having had some glimmers of success with iron saturate glazes in reduction cooling environments, I proposed a solution that did not; 1.) involve expensive silver or bismuth oxides, or caustic salts such as stannous chloride or copper sulfate, and 2.) involve a postfiring or overly exotic and difficult to repeat firing schedule. In the end, a satisfactory solution was some combination of feldspathic glazes with 30-60% Manganese Dioxide, following closely in the steps of David Shaner, Lucie Rie, Hans Coper, John Tilton, and historical Rockingham ware.

Definitions

Reduced-pigment luster. Nearly all historical luster made before 1800 fits in this category. The result of calcining copper, silver, and bismuth oxides with earthenware or laterite clays, and applying the resulting mixture to a maturely fired lead or tin glaze surface. The piece is then refired and held in heavy reduction at dull red heat allowing for a thin layer or metallic oxide to fuse with the surface of the glaze. After the firing, the earthenware is wiped away, revealing a nano-thick layer of iridescent metal.

Resinate luster. Usually made with dissolved gold, platinum, or other noble metals and suspended in an organic binder. Generally fired to a low temperature, with the organic compounds burning out and fluxing a thin, even layer of metallic oxides with the surface of the work. Developed around 1800, very common in industry, very toxic.

Reduced Glaze Luster­. Generally higher porcelain and stoneware temperature. Usually cover the entire surface of a form. Relies on metallic saturated glazes precipitating out thin layers of reduced metallic oxides which deposit in a thin layer on the top of the glaze. Generally more brilliant and operate across a wider spectrum of interrupted light.

Technical Information

            Materials: I found that nearly all of my iridescent surfaces contained some percentage of manganese. The exception is a traditional Tenmoku glaze fired in standard reduction, and then ‘struck’ at 1840F for 1:20-2:00 hours. Strike firing, or striking the kiln is a glass term which refers to increasing the fuel supply and thus creating a reducing atmosphere around 1800F. Initial tests suggested that manganese saturated glazes promoted richer iridescent surfaces regardless of a strike firing. Additions of other oxides were often counterproductive to glossy surfaces and generally resulted in unpleasant black, rough surfaces. Copper, Iron, Chrome, Nickle, and Cobalt were all tested alone and in conjunction from .1 -> 20%. The character of the underlying glass matrix of was usually beer bottle brown, so I tested extensively to change the color of the glass without effecting the iridescent surface – to date I still don’t have a simple solution to this problem. Granular Manganese seemed to produce brighter colors as well as promoting streaking ‘hares-fur’ effects in faster cooling, and acting as ‘seeds’ to crystal formation on slower cooling cycles. My ideal concentration of granular manganese was 2% and fine manganese dioxide at about 27%.

Most recipes called for 50-70% feldspar, and after testing all of the available feldspars, I found that Nepheline syenite promoted a much smoother, regular iridescence. Custer feldspar promoted iridescence across a wider spectrum, but promoted intense crystallization as to appear almost pixellated. Kona f4 promoted a more matte, golden green/purple sheen. Other feldspars promoted a lustrous brown glass with varying degrees of light to moderate iridescence.

The addition of silica promoted a lightening of the glass matrix, as well as a sugary, semi- shiny sparkling satin luster. Silica beyond 15% eliminated iridescence. Alumina additions to the glaze produced a semi-matt honey colored glaze.

I found that the clay body had a huge impact on the color and quality of the iridescence. The most successful clay bodies were grolleg based porcelains, with only the highest percentages of manganese based glaze recipes showing even the slightest luster on stoneware recipes.

Finally, glaze thickness was perhaps the most critical aspect of obtaining iridescence at high temperature. This was complicated as these glazes are extremely runny. Even slight overfiring resulted in glazes running off the pot. There was a need to find a balance between adding clay and silica to the feldspar and manganese without diluting the concentration of available metal oxides and feldspar. It was also extremely difficult to apply these glazes consistently, and fire them in such a way as to reach maturity without overfiring.

Firing: All tests were fired in high temperature gas kilns. I usually fired to 1260C, or Orton cone 10. A majority of my testing was in standard cone 10 reduction firing, with a 1 hour body reduction at cone 012-> cone 08, and a 6-10 hour firing from cone 08-> cone 10. Recipes with 15% copper produced a striking gold color in oxidation environments, and glazes in oxidation firings bubbled and boiled up between cone 7-9, which suggests a similar thermal reduction similar to oil spot glazes.

Cooling: Most of my firings were in small soft brick or fiber kilns, so the possibility of extended cooling cycles was limited. I found that crash cooling seemed to promote smoother, less brilliant surfaces, and a moderately fast cool was ideal in creating a balance between bright color and reasonably smooth surface. Longer cooling promoted larger crystals to a point, and excessively long cooling cycles promoted a matte surface. Reduction cooling remains an exciting possibility which mostly extended beyond the scope of my research. A very interested mottled crystal growth was observed on bottle forms cooled with a 3 hour reduction hold at 1840F.

[1] Caiger-Smith, Alan. Lustre Pottery: Technique, Tradition, and Innovation in Islam and the Western World. London: Faber and Faber, 1985. Print. Pg. 21

[2] Caiger-Smith, 1985, Pg. 177

[3] “Iridescence in Lepidoptera”. Photonics in Nature (originally in Physics Review). University of Exeter. September 1998. Retrieved April 27, 2012.

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Caiger-Smith, Alan. Lustre Pottery: Technique, Tradition, and Innovation in Islam and the Western World. London: Faber and Faber, 1985. Print.Pg 149

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Daly, Greg. Lustre. London: A. & C. Black, 2012. Print.pg. 131

Hamer, Frank, and Janet Hamer. The Potter’s Dictionary of Materials and Techniques. London: & C Black, 1991. Print.