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Over the nickel rainbow: Carol Marians shares her research on her search for nickel blue.

LOOKING THROUGH EMMANUEL COOPER'S The Potter's Book of Glaze Recipes, I became intrigued by the variety of colours in nickel-containing glazes. Blue being the rarest, I set out to find a great nickel blue. Rarely was a glaze journey as full of delightful surprises.

Inspired by Cooper's #94, I designed Ana#1, substituting strontium for barium and soda felspar for potash felspar. In a digital electric kiln, I investigated composition and cool down in achieving nickel blues. Ana#1 applied to tiles, is a variegated, pointillist, temperamental robin s egg blue, [See image captions.] But how different is Ana #1 from all' other glazes? If it is, what makes it so? Where are its neighbouring blue glazes? And what would 'neighbourhood' mean?

Ana #1 stands out because of its 0.1 MgO molecular equivalent. This is a measured, discernible amount of magnesium, not the trace found in all glazes. Nor is it the 0.2 plus magnesium of classical magnesium glazes. It falls in between. Ana#1 is a sensitive multi-phase assemblage, balancing between blue on one side, brown on the other. The volume fraction of matte to glossy varies, as does the colour. The strong blue concentrates in the matte phase, in firings with holds near 1500[degrees]F. The glossy phase is mauve / taupe bordering on tan.

Colour is most intense with thorough intermixing of the two phases, suggesting holds are most relevant just where liquid and solid meet. Examined through a jeweller's loupe, Ana #1 appears an almost fully glossy transparent second phase surrounding islands of sugary white. It is its small crystals that 'carry' the blue colour, so they are wanted in large numbers. If instead, there were a small number of large crystals, a white glaze with a few small scattered deep blue dots would result. Thickly applied, the glaze looks white, not blue.

The needed multitude of small crystals must be well dispersed throughout the glaze to achieve that elusive blue. Therefore a large number of particles must be obtained in the solidification of the glaze, or through slow growth of the second phase, near the end of solidification.

Magnification explains what is going on. At different points in heating, and again in cooling, specific ingredients melt and form rivulets, which may or may not converge with other rivulets. Glaze colours are most often achieved when particular rivulets are 'frozen into the solidified glaze. When glazes 'break' at the edges it is because one 'rivulet' takes over from another. We see the sky as blue because of light rays, bouncing around and hitting molecules of air and bits of dust. Similarly, flint assumes many colours, because of light reflections trapped inside the rock. And the structures and conditions 'trapped' in the glaze give us the colours we see.

My first tests had a blue ground with variegated blue and violet dots. I went on to get many variations: tan, opaque white with blue specks, near gloss translucent taupe with no visible blue. I learned nickel blue requires a glaze composition allowing the formation of the blue coloured material. I had to find the conditions under which the blue particles could be seen.

A glaze's composition determines the starting point from which it assumes characteristics in cooling. A glaze is balanced between opposites: gloss / matte, opaque / transparent, fine on one side / coarse, with a lizard skin surface texture on the other. Fractional solidification during cooling pushes properties from one side to the other. As the glaze heats, solids interact and liquids form. Each reaction occurs at its 'own temperature, so that at the maturing temperature, a melt with no remaining solid results. At top temperature, a fully moulten layer sits on the pot. Were the pot held at this temperature--for hours, even days--changes would continue and segregation and textures might develop. (Hare's fur, partridge feather iron glazes are examples, as are celadons.)

Then, as the glaze cools the first solid forms, changing greatly the composition of what remains liquid. What happens later is determined by what happens first. Empirical formulas tell us glazes are made up of bases, alumina, and silica. The bases divide into alkali: [K.sub.2]O, [Na.sub.2]O, [Li.sub.2] and alkaline earths: CaO, MgO, ZnO and SrO. A glaze's parameters cannot be changed arbitrarily. Working with unity formulas requires the sum of the bases to be adjusted to one and means that when one base is increased, the others must be lowered. Meanwhile, the silica / alumina ratio influences balance of glossy and matt, as well as the maturation temperature. A 'finished' glaze results from a balancing of fluxes (bases) and silica/ alumina.

[FIGURE 6 OMITTED]

I experimented with small variations in the proportions of the bases (CaO/MgO/SrO) changing as little in the empirical formula as possible while maintaining a cone 6 glaze. I focused on MgO and CaO. Lowering magnesium produced a lavender glaze Ana#3 [Figure 6], which seems more stable than the blue in Ana#1. Raising MgO resulted in a brown glaze.

[FIGURE 7 OMITTED]

I checked out the role of strontium. Keeping the same alumina and silica content and proportion meant reducing SrO. This would raise CaO or MgO and might result in a more fluid glaze. I did not want another brown glaze but when I increased the calcium, that is what I had. Within the limits of my tests I never created a fully transparent nickel glaze that was anything but brown.

At this point I looked through all of my glaze notes and chose one from each glaze family. There I found Jon [Figure 7] which has neither SrO nor ZnO. Next I focused on the influence of CaO on the colour of Nickel in a glaze and compared colour to CaO content. Finally I achieved RuthMeske#1 [Figure 8], a saturated teal blue breaking yellow on the edges. It was quite stiff, with high surface tension. I lowered [Al.sub.2][O.sub.3], kept the same Si[O.sub.2]/[Al.sub.2][O.sub.3] ratio to achieve RuthMeske#2 [Figure 9], a smooth teal blue.

[FIGURE 8 OMITTED]

Along the road to Nickel Blue I got the following results. (The starred glazes are those whose results were positive and whose recipes are given here.)

[FIGURE 9 OMITTED]

As can be seen, nickel blue is unlikely in a high CaO glaze. Next I wanted to learn the effect of controlled cooling and 'holds' on the glaze colour. The slower the kiln is cooled, or held at a given temperature, the more change can occur in the glaze. But kiln cooling times are sufficiently rapid that re-absorption of newly formed solids cannot take place. The composition of an already solidified material is set. The rate of cooling can affect glaze only up to complete solidification. For cone 6, 1400[degrees]F seems a reasonable bottom temperature for slow cooling and 2000[degrees]F an equally reasonable top temperature.

A short hold just after the top temperature is reached helps to even out the temperature in the kiln. A long hold at this point will only raise the effective cone of the firing.

From cone 6 (for me 2160[degrees]F) to 1400[degrees]F the following intervals were useful in interpreting my many firings.

1. 2000-1800[degrees]F: The entire glaze is moulten, possible precipitation of crystals as in saturated iron glazes.

[FIGURE 10 OMITTED]

[FIGURE 11 OMITTED]

[FIGURE 12 OMITTED]

[FIGURE 13 OMITTED]

[FIGURE 14 OMITTED]

[FIGURE 15 OMITTED]

2. 1800-1600[degrees]F: In multi-phase glazes, segregation has begun, one or more phases may have begun to solidify.

3. 1600-1400[degrees]F: The last phases present have begun to solidify.

My intervals were determined by dividing the total cooling into three phases. To illustrate the process: Ana#1 in five firings, time given in hours beyond normal kiln cooling.
Region 1   Region 2   Region 3   time above 1400[degrees]F

1. 0          0          0                   0
2. 2.4       1.3        .67                 4.4
3. 0          4          0                   4
4. 0          2          5                   7
5. 0          0          5                   5


Fast cooling #1, gives a semi-transparent, semi-gloss taupe.

Firing #2, retarded moderate cooling gives a tan waxy matte.

Firing #3, with the time concentrated in Region 2, gives a pale waxy matte blue, with a sprinkling of deep blue spots.

Firing #4, more time in Region 3--an exceedingly slow down fire--gives a dry densely opaque matte, with only a sprinkling of dark blue.

Firing # 5 yields that deep blue colour in a semi transparent waxy glaze with a little gloss (Figures 1-5)

Here are my cooling and hold schedules. In these firings only the firing down had any influence. [--] means fire down at a rate of 50[degrees]F per hour in the interval contained between the brackets. Each'*' indicates 15 minutes of hold.

(For example, Firing #4 has a slow down firing from 1700[degrees]F to 1400[degrees]F, with one hour hold at 1500[degrees]F.)

[ILLUSTRATION OMITTED]

Having done all of that, I set out to find how the combination of nickel and other materials would influence colour. I began with ruble which resulted in a gorgeous buttery yellow, Silvia#1 (Figure 10), which started me on further quests to find the end of the Nickel Rainbow. So again using ruble resulted in Silvia#2 and Silvia#3 (Figures 11, 12) and amHi (Figure 13). Adding ruble and iron resulted in am (Figure 14). Last, the glaze loCa gives a fabulous aqua if applied very thick (Figure 15).
GLAZES

Ana#1

Bentonite                4.0
Custer Felspar           5.6
EPK Kaolin               31.7
Lithium Carbonate        2.6
Nepheline Syenite        33.6
Silica                   66.2
Strontium Carbonate      18.7
Talc                     7.2
Unispar                  9.5
Whiting                  9.7
Zinc Oxide               11.7
   Nickel Oxide          2

Chemical Composition:           [K.sub.2]0             0.0498
                                [Na.sub.2]O            0.1195
                                Ca0                    0.2011
                                Mg0                    0.1002
                                [Li.sub.2]O            0.0613
                                Ba0                    0.0000
                                Zn0                    0.2484
                                Sr0                    0.2196
                                [A1.sub.2][0.sub.3]    0.3983
                                [B.sub.2][0.sub.3]     0.0000
                                [Fe.sub.2][0.sub.3]    0.0033
                                Si[O.sub.2]            3.4146
                                [P.sub.2][0.sub.5]     0.0009
                                Zr[O.sub.2]            0.0000
                                Ti[O.sub.2]            0.0027
                                Sn[O.sub.2]            0.0000

Ana#2

Bentonite                 4.0
EPK Kaolin               29.9
Lithium Carbonate         2.6
Nepheline Syenite        13.2
Silica                   64.6
Strontium Carbonate      20.9
Unispar                  40.4
Whiting                  13.2
Zinc Oxide               11.5
  Black Nickel Oxide     2

Chemical Composition:           [K.sub.2]O             0.0502
                                [Na.sub.2]O            0.1205
                                Ca0                    0.2613
                                Mg0                    0.0060
                                [Li.sub.2]O            0.0625
                                Ba0                    0.0000
                                Zn0                    0.2490
                                Sr0                    0.2505
                                [A1.sub.2][0.sub.3]    0.4003
                                [B.sub.2][0.sub.3]     0.0000
                                [Fe.sub.2][0.sub.3]    0.0029
                                Si[O.sub.2]            3.4251
                                [P.sub.2][0.sub.5]     0.0009
                                Zr[O.sub.2]            0.0000
                                Ti[0.sub.2]            0.0026
                                Sn[O.sub.2]            0.0000

RuthMeske#1

Bentonite                 4.0
Custer Felspar            6.4
EPK Kaolin               36.4
Lithium Carbonate         2.4
Nepheline Syenite        36.4
Silica                   71.8
Strontium Carbonate      19.4
Talc                      6.5
Whiting                   2.1
Zinc Oxide               14.9
  Black Nickel Oxide     4

Chemical Composition:           [K.sub.2]O             0.0500
                                Na20                   0.1202
                                Ca0                    0.0701
                                Mg0                    0.0999
                                [Li.sub.2]O            0.0600
                                Ba0                    0.0000
                                Zn0                    0.3497
                                Sr0                    0.2500
                                [A1.sub.2][0.sub.3]    0.4504
                                [B.sub.2][0.sub.3]     0.0000
                                [Fe.sub.2][0.sub.3]    0.0041
                                Si[O.sub.2]            3.8494
                                [P.sub.2][0.sub.5]     0.0012
                                Zr[O.sub.2]            0.0000
                                Ti[O.sub.2]            0.0034
                                Sn[O.sub.2]            0.0000

RuthMeske#2

Bentonite                 4.0
Custer Felspar            7.3
EPK Kaolin               32.0
Lithium Carbonate         2.6
Nepheline Syenite        39.8
Silica                   67.6
Strontium Carbonate      21.2
Talc                      7.2
Whiting                   2.3
Zinc Oxide               16.3
   Black Nickel Oxide     4.0

Chemical Composition:
                                [K.sub.2]O             0.0500
                                [Na.sub.2]0            0.1199
                                Ca0                    0.0699
                                Mg0                    0.0998
                                [Li.sub.2]0            0.0600
                                Ba0                    0.0000
                                Zn0                    0.3496
                                Sr0                    0.2508
                                [A1.sub.2][0.sub.3]    0.3996
                                [B.sub.2][O.sub.3]     0.0000
                                [Fe.sub.2][0.sub.3]    0.0034
                                Si[O.sub.2]            3.4250
                                [P.sub.2][0.sub.5]     0.0009
                                Zr[0.sub.2]            0.0000
                                Ti[0.sub.2]            0.0027
                                Sn[O.sub.2]            0.0000

am

Bentonite               4.0
Custer Felspar          20.3
EPK Kaolin              30.4
Frit 3134               20.9
Lithium Carbonate       11.2
Nepheline Syenite       48.7
Red Iron Oxide          3.1
Rutile                  10.6
Silica                  29.0
Whiting                 22.3
  Black Nickel Oxide    3

Chemical Composition:           [K.sub.2]O             0.0747
                                [Na.sub.2]O            0.1951
                                Ca0                    0.4841
                                Mg0                    0.0067
                                [Li.sub.2]O            0.2395
                                Ba0                    0.0000
                                Zn0                    0.0000
                                Sr0                    0.0000
                                [A1.sub.2][0.sub.3]    0.4203
                                [B.sub.2][0.sub.3]     0.1097
                                [Fe.sub.2][0.sub.3]    0.0331
                                Si[O.sub.2]            2.6017
                                [P.sub.2][0.sub.5]     0.0008
                                Zr[0.sub.2]            0.0000
                                Ti[0.sub.2]            0.2124
                                Sn[O.sub.2]            0.0000

amHi

Bentonite               4.1
EPK Kaolin              21.8
Frit 3124               34.8
Lithium Carbonate       11.1
Nepheline Syenite       61.1
Rutile                  10.6
Silica                  36.3
Whiting                 20.6
  Nickel Oxide          4

Chemical Composition:           [K.sub.2]O             0.0536
                                [Na.sub.2]0            0.2150
                                Ca0                    0.4844
                                Mg0                    0.0070
                                [Li.sub.2]0            0.2399
                                Ba0                    0.0000
                                Zn0                    0.0000
                                Sr0                    0.0000
                                A1.sub.2][0.sub.3]     0.4201
                                [B.sub.2][0.sub.3]     0.1100
                                [Fe.sub.2][0.sub.3]    0.0023
                                Si[0.sub.2]            2.8109
                                [P.sub.2][O.sub.5]     0.0006
                                Zr[O.sub.2]            0.0000
                                Ti[O.sub.2]            0.2142
                                Sn[O.sub.2]            0.0000

Silvia#1

Bentonite               5.9
Custer Felspar          12.3
Lithium Carbonate       5.9
Nepheline Syenite       106.2
Rutile                  8.5
Silica                  32.3
Talc                    6.5
Whiting                 7.2
Zinc_Oxide              5.7
Zircopax                9.8
  Nickel Oxide          2

Chemical Composition:           [K.sub.2]O             0.1211
                                [Na.sub.2]0            0.3263
                                Ca0                    0.1777
                                Mg0                    0.1004
                                [Li.sub.2]O            0.1472
                                Ba0                    0.0000
                                Zn0                    0.1274
                                Sr0                    0.0000
                                [A1.sub.2][0.sub.3]    0.5100
                                [B.sub.2][0.sub.3]     0.0000
                                [Fe.sub.2][0.sub.3]    0.0015
                                Si[0.sub.2]            3.5618
                                [P.sub.2][0.sub.5]     0.0000
                                Zr[O.sub.2]            0.0982
                                Ti[0.sub.2]            0.1963
                                Sn[0.sub.2]            0.0000

Silvia#2

Bentonite               6.4
Dolomite                13.3
Lithium Carbonate       6.5
Nepheline Syenite       125.3
Rutile                  9.3
Silica                  22.9
Zinc_Oxide              6.2
Zircopax                10.6
  Nickel Oxide          4

Chemical Composition:           [K.sub.2]O             0.1041
                                [Na.sub.2]O            0.3400
                                Ca0                    0.1489
                                Mg0                    0.1328
                                [Li.sub.2]0            0.1470
                                Ba0                    0.0000
                                Zn0                    0.1273
                                Sr0                    0.0000
                                [A1.sub.2]0            0.5068
                                [B.sub.2][O.sub.3]     0.0000
                                [Fe.sub.2][0.sub.3]    0.0013
                                Si[O.sub.2]            3.0011
                                [P.sub.2][0.sub.5]     0.0000
                                Zr[O.sub.2]            0.0979
                                Ti[O.sub.2]            0.1959
                                Sn[0.sub.2]            0.0000

IoCa

Bentonite               4.0
Custer Felspar          144.0
EPK Kaolin              10.4
Lithium Carbonate       4.8
Silica                  6.0
Talc                    8.3
Whiting                 4.3
Zinc_Oxide              18.5
Nickel Oxide            4

Chemical Composition:           [K.sub.2]O             0.2442
                                [Na.sub.2]0            0.1081
                                Ca0                    0.0979
                                Mg0                    0.0991
                                Li20                   0.1003
                                Ba0                    0.0000
                                Zn0                    0.3503
                                Sr0                    0.0000
                                [Al.sub.2][O.sub.3]    0.4513
                                [B.sub.2]O.sub.3]      0.0000
                                [Fe.sub.2][0.sub.3]    0.0026
                                Si[O.sub.2]            3.0195
                                [P.sub.2][0.sub.5]     0.0003
                                Zr[O.sub.2]            0.0000
                                TiO                    0.0009
                                Sn[O.sub.2]            0.0000

Silvia#3

Bentonite               4.99
Custer Felspar          97.4
Dolomite                21.8
Lithium Carbonate       9.8
Nepheline Syenite       53.9
Rutile                  10.6
Talc                    1.47
Black Iron Oxide        6
Nickel Oxide            4

Chemical Composition:           [K.sub.2]O             0.2000
                                [Na.sub.2]0            0.2004
                                Ca0                    0.1996
                                Mg0                    0.2003
                                [Li.sub.2]0            0.1997
                                Ba0                    0.0000
                                Zn0                    0.0000
                                Sr0                    0.0000
                                [Al.sub.2][O.sub.3]    0.4504
                                [B.sub.2]O.sub.3]      0.0000
                                [Fe.sub.2][0.sub.3]    0.0016
                                Si[O.sub.2]            2.6042
                                [P.sub.2][0.sub.5]     0.0000
                                Zr[O.sub.2]            0.0000
                                Ti[0.sub.]             0.1998
                                Sn[O.sub.2]            0.0000

Ion

Bentonite               4.0
Dolomite                8.4
EPK Kaolin              12.5
Frit 3134               3.1
Lithium Carbonate       4.8
Nepheline Syenite       86.6
Silica                  67.7
Whiting                 13.3
Nickel Oxide            4

Chemical Composition:           [K.sub.2]O             0.0855
                                [Na.sub.2]0            0.2866
                                Ca0                    0.3987
                                Mg0                    0.0998
                                [Li.sub.2]0            0.124
                                Ba0                    0.0000
                                Zn0                    0.0000
                                Sr0                    0.0000
                                [Al.sub.2][O.sub.3]    0.5020
                                [B.sub.2]O.sub.3]      0.0199
                                [Fe.sub.2][0.sub.3]    0.0023
                                Si[O.sub.2]            4.3097
                                [P.sub.2][0.sub.5]     0.0004
                                Zr[O.sub.2]            0.0000
                                Ti[0.sub.2]            0.0013
                                Sn[O.sub.2]            0.0000


[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

Carol Marians has been a potter since 1952 when she discovered clay in a craft course. She proceeded to earn two PhDs from MIT, one in Math, the other in Materials Science. Self-trained in clay, she studied the literature and took workshops with F Carlton Ball and Tom Coleman. She has published in Ceramics Monthly and on the web site: http://vickihardin.com/articles/index.html. She currently is devoting her time to the study of glaze. (http://carol.knighten.org/)

I would like to acknowledge Jon Singer, of the Joss Research Institute for many helpful conversations on technical issues.

Photos by Dr Robert Knighten (rlk@knighten.org)

Internet versions of the figures for this article are found at: http://carol.knighten.org/glaze/
TABLE 1: Mol Fraction Calcia

0.030000 brown mauve tinge   0.390000 coffee brown
*0.150000 pale blue          0.400000 pale grey blue tinge
*0.180000 chartreuse         0.420000 greyed brown
0.200000 pale blue           0.420000 pale grey
0.200000 pinkish brown       0.420000 grey flecks blue
0.210000 pale blue           0.480000 pale yellow
*0.270000 mauve              0.500000 pale grey
0.370000 brown               0.510000 light coffee
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Author:Marians, Carol
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Date:Nov 1, 2009
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