Books for Glaze Prospectors

Without a doubt, Pioneer Pottery is the best resource for you diggers out there. Micheal Cardew was an absolutely incredible potter and glaze wizard. Here’s the book summary from Amazon:

You can find it here.

Pioneer Pottery is one of the classic texts in ceramic literature. Its author, Michael Cardew, was one of the seminal figures in British ceramics and this book, written partly in response to his time spent in Africa, has influenced generations of potters around the world.
As one of the early and influential figures in British ceramics, Michael Cardew was asked in 1942 by the Crown Agents to go to Nigeria to set up a pottery training center. It was his experiences there, as well as his previous time spent in the Gold Coast, that formed the basis for this book. In addition to discussions about the basics of clay and glaze materials and their properties, Cardew shows how to make pots in the Kwali, Jebba, Vume, Ilorin, and Hausa traditions of western Africa. This book is the first book to discuss all aspects of the pottery making process from materials through marketing. Michael Cardew found it necessary to find out and do everything for himself, and this is what lies behind this book. The consequence is that it is filled with a sense of discovery and firsthand knowledge.

A second amazing book is Glazes From Natural Sources. I like Sutherland’s perspective, and his instructions on building a home made ball mill and pugmill are super interesting. Although US buyers will probably find this one a little more expensive, it’s a great resource to have. Here’s the Amazon summary:

Glazes from Natural Sources: A Working Handbook for Potters, 2nd edition

Any real education in ceramics must involve, from the outset, an appreciation of the source materials—the rocks around us. While good, throwable clay may be a regional commodity, there is no part of the world that will not yield potential glaze materials in abundance. Potters therefore need to know how to exploit the special properties of local rocks quickly, reliably, and methodically. This new revised and updated version of Brian Sutherland’s classic book on making glazes from natural sources explains how to locate glaze material and construct, test, and use the glazes created.

Glazes from Natural Sources discusses rock types and other likely sources of supply, the making of test pieces, and the use of blend systems and constructions. The author also clarifies the Seger system of glaze presentation for those who, like himself, have found it difficult to grasp and apply. He covers, and supplies, formulae for glazes for all temperatures—from raku to stoneware and porcelain—and includes sample recipes. The book emphasizes careful planning and control to ensure results that are repeatable. This makes the science behind making glazes from natural sources both understandable and feasible.

Glazes from Natural Sources is fully illustrated, with diagrams of techniques demonstrated as well as images of the finished works of potters to show the results of these natural glazes. First published in 1987, this book is considered a classic by ceramicists. This edition includes a new chapter by glaze expert Nigel Wood.

Basalt as Colorant in Celadon Glazes

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 (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.

 

Basalt from 0-10%

TCB10

Mn Dark Stoneware with 10% Basalt Chun Left, 10% Basalt Celadon Right

Celadon with 0-10% Basalt

Basalt Celadon 7.5%, 10%

Basalt Celadon 0%, 2.5%, 5%

Liquor Bottle Set

Bottle, Cups, Porcelain, 2015

Rhyolite and Basalt Glazes

I was beyond excited to work with my newest found material, a rhyolite from Topaz Mountain, in Juab Country, Utah.  This time rather than choosing a handful of very large rock samples (to insure relative material consistency), I instead went to a wash and filled up a 5 gallon bucket with very fine material the size of course sand. My reasoning this time was that consistency is completely relative, and as long as I get materials from the same spots, it doesn’t matter – and I can grab material that has already been 99% processed for me. In the end I think this worked out, because I was able to run 5 gallons of sand through our ball mill with 2x 1 gal. ball mill jars in 10 batches. But I’m getting a bit ahead of myself, because I think it’s important to test fire a material before you go through the trouble of ball milling. So my new first step in dealing with materials (after identification of course) is to take a small chunk, put it in a small dish, and fire to cone 10 in reduction. Since this is my primary temperature range, that’s it, if there are chances I’ll also put similar samples into cone 6 oxidation as well as an oilspot firing schedule, which is about cone 12 oxidation. Here was the result at cone 10, in reduction:

A small rhyolite pebble after being fired to cone 10, in reduction.

 

Looks a lot like a fired chunk of granite or feldspar. Onwards with the milling!

Someone asked me about my process for ball milling, and here it is: Fill a 1 gal ball mill jar 1/3 with mixed sized media (approx 50% 1/4″ balls, 25% 1/2″ balls, 25% 1″ balls) then fill the jar with 1/2 gallon of water, then fill the the rest of the container up with material until it’s about 2/3 full.) If I had more containers I wouldn’t exceed filling the jar 1/2 way, but my circumstances are what they are, and I haven’t needed to change anything yet, such as it is.

In reduction, this rhyolite material was surprisingly similar to my ice crackle glaze. I think with very little modification (a small addition of clay, bone ash, and maybe a bit of frit) I’m nearly positive this will look and feel like a Kuan, ice crackle glaze.

Rhyolite Glaze on a high Iron clay body. Fired to cone 10 in Reduction.

Once I had all of my material milled, I let it sit overnight and then drained off the excess water, leaving me with a glaze slurry with an SPG of 1.58 (That’s 79g of material in a 50cc syringe). That’s only important if you want to know how much material you have per given volume. Since I was going to blend this with a basalt material that was also in solution, I needed this info. After taking the SPG of my basalt material, which happened to be 1.54, I did a simple line blend. On both sides are the materials by themselves, in the middle a 50/50, and on the left and right middle 25/75.

Rhyolite/Basalt line blend. Red stoneware (top row) and porcelaineous (bottom). Fired to cone 12, oxidation.

 

Pretty interesting results, I think. The big surprise was how sweet the 25% Basalt and 75% Rhyolite mix came out.

1 part Basalt, 3 parts Rhyolite. Fired to cone 12 oxidation.

Finally, because I was looking for an oilspot/tenmoku type glaze with this research, I should also detail my firing schedule. Here’s my current Blaauw gas kiln firing schedule:

0	time_temp 00:00 5
1	time_temp 01:30 200
2	time_temp 07:00 1160
3	time_temp 01:30 1200
4	time_temp 01:00 1220
5	time_temp 02:00 1230
6	time_temp 01:15 1252
7	oxidation 80
8	time_temp 00:08 1252
9	oxidation 150
10	time_temp 00:30 1220
11	time_temp 01:30 1200
12	cooling
13	time_temp 02:00 1000
14	time_temp 02:00 800
15	time_temp 02:00 700
16	time_temp 02:00 500
17	time_temp 02:00 300
18	time_temp 02:00 50
19	time_temp 04:00 50

Blaauw kilns have the capability of firing in extremely oxidized conditions – blowing in somewhere to the tune of double the amount of air needed for complete combustion. The default, and maximum air value is 200. An neutral flame is around 100, and a smoky reduction is something like a 70.

Basically, this program fires up to cone 6 in about 9 hours, and then goes slowly up to 1252C, reduces for 8 minutes, and then goes back to oxidation, drops to 1220 over the course of 30 minutes, then drops to 1200 over the course of an hour and a half.  I’m still very much tweaking this schedule, which works very well for some glazes, and not so much for others.

Rhyolite From Topaz Mountain, UT

View From Topaz Mountain, Juab County, Utah. Photo by Phil Konstantin

A few weeks ago the USU Mineralogy class took an overnight field trip to Topaz Mountain in Juab County, Utah. This location is known for an abundance of semi-precious gemstone, namely a champagne colored topaz, opal, and red beryl. Unfortunately, the topaz loses its color after exposure to UV radiation (sunlight) so the gemstones, although beautiful, aren’t super valuable.

Of more interest to me, of course, was the rhyolite material itself. After working quite a bit with the ultramafic (high in magnesium and iron) basalts from the Snake River Plain in Idaho, I was coming to the conclusion that I needed to add in silica and alumina to stabilize this glaze and keep it from flowing off of my pots as well as having a nice and glossy glaze surface. Quite by luck, I was in the perfect spot to find a material that was precisely what I needed to mix together with my basalt material to get something interesting.

In the end, I had a lot of fun busting open rocks and attacking the rhyolite outcrops with a 5 pound sledge. I took some pictures of some of the coolest, and largest topaz pockets, which are referred to as “vugs”. At the end of the day I filled up a 5 gallon bucket with this material and brought it back to the studio to go straight into the ball mill. More on the results in a later post!

 

A vug I unearthed filled with Amber Topaz.

More vugs filled with the distinct champagne colored Topaz. Unfortunately these topaz lose color with exposure to sunlight.

 

From the paper ‘Geochemical evolution of topaz rhyolites from the Thomas Range and Spor Mountain, Utah. – Christiansen, Bikun, Sheridan, Burt, 1984

 

Zinc Things

Zinc

I got a lot of responses after my last post, where I made the claim that using zinc is hard on elements. I had some really interesting discussions with both John Britt and John Tilton on this subject, and I thought I’d share some of that info with you guys. Of particular interest was a tidbit from Britt where he mentioned that firing a zinc sample in oxidation leaves it unmelted (as the melting temp is 3500) while the zinc sample in reduction almost entirely goes away. Heres a fired sample from our glaze calc class, fired in cone 10 reduction. It was a thumbnail sized lump when it went in.

Zinc Oxide

To make a long story short, the juries out on whether or not zinc directly reduces the life of kiln elements. What’s most likely is that it’s a combination of complex volatilizing compounds released during the firing coupled with high temperatures (especially if there are programmed holds). An often overlooked variable is water. It could very likely be that the extremely hard water at my current and previous studios is the culprit. If you’ve ever looked closely at old workhorse kilns, some of the deposits near the lids and spy ports are similar to calcite deposits. Its not hard to imagine Calcium and Flouride attacking the elements.

Another variable to consider is that the kilns that I’ve had access to for my entire career have been heavily used community kilns. At research universities there’s a huge range of clays and glazes that go through kilns. I’ve also heard that barium, cobalt, and copper are also hard on elements. Given all these extra variables, I suppose the only real way to quantify if and how much zinc affects elements would be to fire two brand new kilns side by side for a 100 or so firings and then compare. Let me know if you have two brand new kilns you need me to test. I’ll be happy to help you out!

On a related note, John Tilton had some great info on kiln elements. I hope he doesn’t mind me sharing the info – it seems very useful. From an email from Tilton:

The zinc thing is somewhat solved by using heavier elements. I have 11 gauge KA1s in my newest L&L, and after 104 firings they are still standing nicely. 12 gauge seems to give about 200 firings, and 13 gauge 80. Normal elements, the 16 gauge or so, give between 18 and 35 firings, not worth using. I also fire only one or two pots at a time so the total zinc load per firing is probably less than if you stacked tightly.

So it looks like 11 gauge might be the sweet spot, though they do require length to be resistant enough, and maybe that translates into coils of larger diameter.

Finally, I’ll leave you with the Material description from Digital Fire. I highly recommend consulting digital fire for material descriptions, and if you have the means, purchase access to the software. So worth it. From Digitalfire.com

Zinc Oxide

Pure Source Of Zinc

Formula: ZnO
Alternate Names: ZnO, Zincite

Oxide Analysis Formula ZnO 100.00% 1.000 Oxide Weight 81.40 Formula Weight 81.40

Enter the formula and formula weight directly into the Insight MDT dialog (since it records materials as formulas). Enter the analysis into an Insight recipe and enter the LOI using Override Calculated LOI (in the Calc menu). It will calculate the formula.

DENS – Density (Specific Gravity)

5.6

Zinc oxide is a fluffy white to yellow white powder having a very fine physical particle size (99.9% should pass a 325 mesh screen). It is made using one of two processes that produce different densities. The French process vaporizes and oxidizes zinc metal, the American process smelts a coal/zinc sulfide mix and oxidizes the zinc fumes.

Ceramic grades are calcined, they have a larger particle size and much lower surface area (e.g. 3 square meters per gram vs. less than 1; however 99.9% still passes 325 mesh). Calcined grades are said to produce less glaze surface defect problems (although many ceramists have used the raw grades for many years without serious issues). You can calcine zinc on your own in a bisque kiln, fire it at around 815C. Calcined zinc tends to rehydrate from atmospheric water (and get lumpy in the process, calcining a mix of zinc and kaolin produces a more workable powder). Alot of zinc is used in crystalline glazes (typically 25%), because these have no clay content, they bring out the best and worst of both the calcined and raw materials. The raw zinc suspends glazes much better (the calcined settles out much more). The raw zinc takes more water, but since the glaze can thin out over time it is better to add less than needed at mixing time and mix thoroughly. The raw zinc screens better (although it can be a challenge to get either slurry through an 80 mesh screen).

Zinc oxide is soluble in strong alkalies and acids.

It can be an active flux in smaller amounts. It generally promotes crystalline effects and matteness/softness in greater amounts. If too much is used the glaze surface can become dry and the heavily crystalline surface can present problems with cutlery marking. Other surface defects like pitting, pinholing, blistering and crawling can also occur (because its fine particle size contributes to glaze shrinkage during drying and it pulls the glaze together during fusion).

Zinc oxide is thermally stable on its own to high temperatures, however in glazes it readily dissolves and acts as a flux. Zinc oxide sublimes at 1800C but it reduces to Zn metal in reduction firing and then boils at around 900C (either causing glaze defects or volatilizing into the atmosphere; note that electric kilns with poor ventilation can have local reduction).

While it might seem that zinc would not be useful in reduction glazes, when zincless and zinc containing glazes are compared it is often clear that there is an effect (e.g. earlier melting). Thus some zinc has either remained or it has acted as a catalyst.

The use of zinc in standard glazes is limited by its price, its hostility to the development of certain colors and its tendency to make glazes more leachable in acids (although zinc itself is not considered a hazardous substance).

Zinc oxide is used in glass, frits, enamels and ferrites. Zinc oxide is also used in large quantities in the rubber and paint industries; in insulated wire, lubricants, and advanced ceramics.    Credit: Tony Hansen

Thanks to Tony, and John Tilton. Also a special thanks to John Britt for the feedback on my last post; the clarification on Feldspar (I was mistaking K200 for Minspar 200 – ignorant to the fact that K200 has been out of production for 20 years!) I’ve since edited my post on Cone 6 with this info.

New Pots, New Recipes

#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 19^th 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.

Bibliography:

Britt, John. The Complete Guide to High-fire Glazes: Glazing & Firing at Cone 10. New York: Lark, 2004. Print.

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

Conrad, John W. Black Pearl and Other Saturated Metallic Glazes. Santa Ana, CA: Falcon Division of Aardvark Clay, 2010. Print.

Currie, Ian. Revealing Glazes Using the Grid Method. Australia: Bootstrap, 2000. Print.

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.

Slides from Presentation

Blue Ice Crackle

Matt Fiske – Blue Engobe Crackle Bowl