Formulating a Clear Glaze Compatible with Chrome-Tin Stains

Description

In ceramics color is often a matter of chemistry, that is, the host glaze must be compatible and have a sympathetic chemistry for the stain being added. Chrome-tin stains are a classic example.

Article

Coming up with a pink glaze may sound like an easy matter. All you do is find a good transparent glaze for the needed temperature, mix in 5%-10% pink stain, and you have it; right?

In the real world that most of us live in, things don't work out quite that well. While it is possible to obtain some sort of blue, green or brown glaze using almost any transparent recipe and the appropriate stain, chrome-tin pink stains do not work this way. The color will only develop in a melt that has a chemistry that is sympathetic to the specific mechanisms under which the color develops.

In a quest for a transparent recipe that will perform well with chrome-tin colors like lilac, coral, maroon, and pink, potters have traditionally used the "text-book junkie" approach. With this method, one rounds up tons of transparent recipes and tests each until both the correct surface quality and color are achieved (you also wind up with a room full of materials you will never use again). This method may succeed on the first test but it may not work until the 101st!

What we need is a more thoughtful, fast-track approach, leaving the trial and error stuff for fine-tuning the final product. We also need to understand why the final recipe works, and even better, why others are unsuccessful.

The key to solving these problems is to adopt the Kiln God's formula viewpoint. He builds ceramic structures from oxides like calcia, soda, alumina, and silica. Again, he obtains the oxides from the materials we supply in a glaze and ,for most purposes, does not care what material sources a particular oxide. It is reasonable then, to appraise fired results on the basis of the oxide formula, not the recipe of powders used to mix the raw glaze.

My first step in making the pink work was to mix the stain into my favorite cone 6 transparent glaze and fire a test. Of course, the result was a disaster; it was gray instead of pink! The question was: Why? The secret to the answer lies in examining the formula, not the recipe. First, I calculated the formula using desktop INSIGHT (of course the same can also be done in an account at the newer Insight-live.com).

Following are the results of the calculation in a detailed format.

DETAIL PRINT - 3134 Clear Glaze
MATERIAL               PARTS  WEIGHT   CaO*  Na₂O*   B₂O₃  Al₂O₃   SiO₂
WEIGHT OF EACH OXIDE                   56.1   62.0   69.6  102.0   60.1
-----------------------------------------------------------------------  MATERIAL
Expan OF EACH OXIDE                    0.15   0.39   0.03   0.06   0.04   Cost/kg
-----------------------------------------------------------------------  -------
 FERRO FRIT 3134....   50.00  190.64   0.18   0.08   0.17          0.39     2.49
 KAOLIN.............   30.00  258.14                        0.12   0.23     0.24
 SILICA.............   20.00   60.00                               0.33     0.19
-----------------------------------------------------------------------  -------
TOTAL                 100.00           0.18   0.08   0.17   0.12   0.95     1.36
UNITY FORMULA                          0.68   0.32   0.63   0.44   3.64
PER CENT BY WEIGHT                    10.43   5.43  11.99  12.36  59.79
Cost/kg  1.36
  Si:Al  8.21
 SiB:Al  9.63
  Expan  6.89

Next, I phoned the stain manufacturer, Mason in this case, and asked what to do. Fortunately, all answers were given from the formula viewpoint. They told me that the CaO content had to be at least 10% but that 15% would be better to assure color development. B₂O₃ should not be too high and a little extra CaO would help counteract the solvent action of B₂O₃ on the color development. I also checked a few textbook indexes under the topic "Chrome Tin Pink" and found a reference in Parmelee that confirmed this. It got more specific, stating that "Calcium oxide tends to counteract the ill effects of B₂O₃ , particularly when the ratio of CaO:B₂O₃ can be at least 3:1". Little bits of light were appearing. These facts seem obvious now but they are a commentary on how helpless we sometimes are.

I increased the calcium oxide, then reduced the B₂O₃ formula value to 1 / 3 of the CaO amount. Now that a major flux had been reduced in the glaze, I also determined that it would be necessary to cut the SiO₂ and Al₂O₃ to harmonize with more traditional cone 5 limits (although I was conscious of needing the SiO₂ to keep the expansion down and retain the gloss). Also I introduced a little MgO to help reduce higher expansion caused by the loss of Al 2O₃and SiO₂ .

The calculations produced a new recipe as follows.

 WHITING           15.00  CaO    0.75*
 FERRO FRIT 3134   25.00  MgO    0.12*
 KAOLIN            20.00  Na₂O   0.13*
 SILICA            20.00  B₂O₃   0.26
 TALC               5.00  Al₂O₃  0.24
                ========  SiO₂   2.29
                   85.00
Cost/kg  0.87
  Si:Al  9.49
 SiB:Al 10.56
  Expan  6.97

With bated breath and unwarranted confidence, I tested this glaze. The results: Complete failure! No color at all! Ah, but I was learning! The 3:1 CaO:B₂O₃ and 15% CaO alone were not enough to achieve the color. What is more, the glaze was crazing badly. The measures I had taken to make up for the loss of SiO₂ and Al₂O₃ were not enough. My 'Parmelee' textbook also mentioned that the SiO₂ has a stabilizing influence and can also reduce the solvent bleaching action of B₂O₃ on the development of color.

I ran another test, increasing the SiO2 and MgO to lower the expansion.

 WHITING           14.90  CaO    0.69*
 FERRO FRIT 3134   28.60  MgO    0.17*
 KAOLIN            22.30  Na₂O   0.13*
 SILICA            26.30  B₂O₃   0.26
 TALC               8.00  Al₂O₃  0.24
                ========  SiO₂   2.53
                  100.10
Cost/kg  0.85
  Si:Al 10.62
 SiB:Al 11.72
  Expan  6.65

What were the results? Again, disaster! This time, I even lost the gray color, ending up with nothing but a clear glaze even with 10% pink stain! There is no question that I was learning. As Thomas Edison said when asked what he had learned in life: "I have discovered thousands of things that don't work!"

Well, back at the textbooks, I kept reading about the detrimental effects of zinc on this type of glaze, however, there was no zinc to remove.

Then I found another clue. One short sentence in the middle of a 6-page marathon of chemistry on chrome-tin stain topics said: "This stain also is unstable in the presence of MgO." I had introduced this very oxide early on to help keep the expansion down and had increased it in a subsequent test.

Few things disagree with magnesia but it was worth a try. So I removed the MgO and replaced it with even more CaO.

Here is how the recipe looked

Pink Glaze Base #4
==================
 WHITING.............   37.70  18.85%
 FERRO FRIT 3134.....   55.80  27.90%
 KAOLIN..............   35.50  17.75%
 SILICA..............   50.70  25.35%
 CORNWALL STONE......   20.30  10.15%
                     ========
                       200.00

                    CaO  0.84* 18.29%
                    MgO  0.00*  0.02%
                    K₂O  0.01*  0.49%
                   Na₂O  0.15*  3.63%
                  Fe₂O₃  0.00*  0.03%
                   TiO2  0.00   0.01%
                   B₂O₃  0.26   7.18%
                  Al₂O₃  0.24   9.70%
                   SiO₂  2.59  60.65%

                Cost/kg  0.81
                  Si:Al 10.61
                 SiB:Al 11.69
                  Expan  7.23

Guess what? I hit the jackpot! This time 10% pink stain gave color that would knock your eyes out. However, there was some crazing on porcelain, but this was not a problem since the glaze melts well and thus there is room for an increase in SiO₂ or the B₂O₃ :Al₂O₃ :SiO₂ complement, neither of which should affect color.

This project really demonstrated that you can learn much when something is unsuccessful. Every time a fired product comes out of the kiln, whether success or failure, you can relate its appearance to the formula and learn something valuable.

Related Information

G1214M, W, N, O and S with Mason stains

This shows clearly how well the M version works with a chrome-tin stain compared to the others. However the 6100 brown stain works best in the N recipe (which have MgO). Notice also that the M has a higher thermal expansion than the others.

A glaze incompatible with chrome-tin stains (but great with inclusion stains)

Left: A cone 6 matte glaze (G2934 with no colorant). Middle: 5% Mason 6006 chrome-tin red stain added. Right: 5% Mason 6021 encapsulated red stain added. Why is there absolutely no color in the center glaze? This host recipe does not have the needed chemistry to develop the #6006 chrome-tin color (Mason specifies 10% minimum CaO, this almost has enough at 9.8%, but it also has 5% MgO and that is killing the color). Yet this same glaze produces a good red with #6021 encapsulated stain at only 5% (using 20% or more encapsulated stain is not unusual - so achieving this color with only 5% is amazing).

Ravenscrag Cone 6 white glaze with 10% Mason chrome tin stain

The body is Plainsman M340 and these two glazes are based on the GR6-A recipe (Ravenscrag Slip + 20% frit). The GR6-C creamy white glaze adds 10% Zircopax to opacify it. The pink version, our code number GR6-L, adds Mason 6006 stain instead. The GR6-A base is zinc-free and just hits the 10% minimum CaO recommended to get color development with a chrome tin stain. This recipe also couples a low MgO level (MgO can kill the color in chrome tin stains).

Bad and good glaze application: The difference was the rheology.

This is GR6-L, is the standard GR6-A Ravenscrag Slip cone 6 base recipe + 10% chrome tin stain (the body is Midstone, the inside glaze is G2926B, the firing schedule is C6DHSC). Chrome tin stains are picky about their host glaze, if it does not have a compatible chemistry they fire grey. Obviously, there is a love affair going on here! But the mug on the left has an issue. The glaze on the left has gone on in varying thicknesses and these are producing crystallizations and runs and the incising is not being highlighted. The one on the right is under control. What is the difference? The rheology of the slurry for the bad mug was wrong - the specific gravity was too high (the water content was too low). Even on a quick dip it was building thickness unevenly and way too fast. And there were drips that were so big they had to be shaved off with a knife! After the addition of a lot of water, to take the specific gravity from 1.55 to 1.45 it was watery enough to accept some Epsom salts to make it thixotropic. The difference was amazing, it went on totally smooth without a single drip, producing the result on the right.

Links

Materials	Stain
Troubles	Chrome Flashing in Ceramic Glazes The development of chrome tin pinks is a combination of the stain and the right chemistry in the host glaze. Unwanted pink flashing occurs where there is a hostile chemistry in the glaze.
Oxides	SnO2 - Tin Oxide, Stannic Oxide
Oxides	Cr2O3 - Chrome Oxide
Projects	Stains

By Tony Hansen
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