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Adjusting Glaze Expansion by Calculation to Solve Shivering

Section: Glazes, Subsection: Thermal Expansion

Description

This page demonstrates how you might use INSIGHT software to do calculations that will help you increase the thermal expansion of a glaze while having minimal impact on other properties.

Article Text

Shivering and crazing are probably the two most common glazing problems in industry and hobby ceramics. When one becomes familiar with some thermal expansion theory and how easily crazing can be dealt with, it becomes a mystery why others seem puzzled by the problem. I have watched people who are in love with one touchy glaze try a myriad of different bodies, alter firing curves in every possible way, and yet, achieve only limited success. And then, there are the "snake oil" remedies. "Just add some silica to stop crazing and take some out to stop shivering" it says in many textbooks. This sounds fine at first but as already discussed, you will likely upset the oxide balance and probably kill its surface character if you add enough to fix the problem. Each glaze has its own personality and complexities, a "one solution fits all" method will not work.

Until now, calculating glaze formulas and their expansion values has been a little tedious to say the least. INSIGHT and similar software changed that, providing instant expansion results for any formula or recipe.

Below is a set of source figures I have chosen to use. As you will see, using the calculation approach is an exercise in relating one glaze to another, one oxide to another. It is not necessary to know what a crazing glaze's expansion is; all we really need to know is whether and adjustment to its recipe will give it a lower expansion to make it fit better onto the body. While we could get into the units of these numbers, this is not really important, their relationship to each other is the key.

Expansion Values
BaO 0.13 K2O 0.33 MnO -0.09
CaO 0.15 Na2O 0.39 TiO2 0.14
PbO 0.08 ZnO 0.09 B2O3 0.03
MgO 0.03 Fe2O3 0.13 Al2O3 0.06
        SiO2 0.04

From the numbers above, it becomes obvious why the addition of silica has a lowering effect on a glaze's expansion (although not to the degree you might think). One also begins to appreciate why high-temperature glazes (which have so much more SiO2 and Al2O3) are much easier to fit to clay bodies without crazing. Yes, SiO2 has a very low expansion, so glazes containing plenty of it will tend to have a low expansion also and thus resist crazing. However there is a caveat to this: Even high silica glazes will craze if they contain significant amounts of Na2O and K2O (from feldspar, for example).

Let's look at a shivering problem that occurred in one studio. Below is an INSIGHT report of the original shivering glaze.

DETAIL PRINT - Matte White Glaze
MATERIAL               PARTS  WEIGHT   CaO*   MgO*   K2O*  Na2O*   ZrO2 Fe2O3*   TiO2  Al2O3   SiO2
WEIGHT OF EACH OXIDE                   56.1   40.3   94.2   62.0  123.2  160.0   79.7  102.0   60.1
-- -------------------------------------------------------------------------------------------------  MATERIAL
Expan OF EACH OXIDE                    0.15   0.03   0.33   0.39   0.02   0.13   0.14   0.06   0.04   Cost/kg
---------------------------------------------------------------------------------------------------  -------
 CUSTER FELDSPAR....   52.50  617.10   0.00          0.06   0.03          0.00          0.09   0.60     0.00
 KAOLIN.............   15.00  258.14                                                    0.06   0.12     0.24
 FLINT..............   14.50   60.00                                                           0.24     0.19
 WHITING............    9.50  100.00   0.09                                                             0.12
 TALC...............    5.00  126.23          0.04                                             0.05     0.25
 SUPERPAX...........    1.50  183.00                               0.01                        0.01     0.00
 BENTONITE..........    2.00  489.48   0.00   0.00   0.00                 0.00   0.00   0.00   0.02     0.00
---------------------------------------------------------------------------------------------------  -------
TOTAL                 100.00           0.10   0.04   0.06   0.03   0.01   0.00   0.00   0.15   1.04     0.09
UNITY FORMULA                          0.44   0.18   0.26   0.11   0.04   0.01   0.00   0.68   4.70
PER CENT BY WEIGHT                     5.87   1.76   5.72   1.69   1.08   0.22   0.00  16.45  67.20
Cost/kg  0.09
  Si:Al  6.93
 SiB:Al  6.93
  Expan  6.90

Notice that the expansion is calculated based on the figures shown under each oxide column title. It is very clear which oxides need to be increased to move the expansion up. Shivering is much less common than crazing, so it is not likely that too much change is needed. It should be possible to leave the SiO2 and Al2O3 alone and thereby minimize fired property changes associated with disruptions in the overall balance.

If exotic color compatibility is not at issue (it depends on the absence or presence of certain fluxes), a conservative starting point is to redistribute the fluxes, increasing one at the expense of another. As noted, sodium has the highest expansion, so it is logical to increase it at the expense lower ones like CaO and MgO. You have probably noticed that high sodium glazes (ones with plenty of nepheline syenite and soda feldspar) tend to craze. However Na2O contributes properties much more similar to K2O, therefore I will first try to remove Na2O and replace with an equal amount of K2O.

A common sodium sourcing material, as mentioned, is nepheline syenite. Using INSIGHT, I made a quick substitution for potash feldspar, then juggled remaining material amounts to compensate for the differences in these two materials. Here is the result.

DETAIL PRINT - Matte White Glaze
MATERIAL               PARTS  WEIGHT    CaO    MgO    K2O   Na2O   ZrO2  Fe2O3   TiO2  Al2O3   SiO2
WEIGHT OF EACH OXIDE                   56.1   40.3   94.2   62.0  123.2  160.0   79.7  102.0   60.1
---------------------------------------------------------------------------------------------------  MATERIAL
Expan OF EACH OXIDE                    0.15   0.03   0.33   0.39   0.02   0.13   0.14   0.06   0.04   Cost/kg
---------------------------------------------------------------------------------------------------  -------
 NEPHELINE SYENITE..   49.30  446.40   0.01   0.00   0.02   0.08                        0.11   0.50     0.33
 KAOLIN.............   10.81  258.14                                                    0.04   0.08     0.24
 FLINT..............   22.70   60.00                                                           0.38     0.19
 WHITING............    9.00  100.00   0.09                                                             0.12
 TALC...............    4.60  126.23          0.04                                             0.05     0.25
 SUPERPAX...........    1.50  183.00                               0.01                        0.01     0.00
 BENTONITE..........    2.00  489.48   0.00   0.00   0.00                 0.00   0.00   0.00   0.02     0.00
---------------------------------------------------------------------------------------------------  -------
TOTAL                  99.92           0.10   0.04   0.02   0.08   0.01   0.00   0.00   0.16   1.04     0.25
UNITY FORMULA                          0.10   0.04   0.02   0.08   0.01   0.00   0.00   0.16   1.04
PER CENT BY WEIGHT                     5.74   1.66   2.49   5.18   1.07   0.08   0.00  17.41  66.37
Cost/kg  0.25
  Si:Al  6.47
 SiB:Al  6.47
  Expan  7.17

Notice that the calculated expansion is up modestly to 7.2 (from about 7.0). The relative amounts of the fluxing oxides (refer to the "UNITY FORMULA" line) have altered somewhat, but the SiO2 and Al2O3 remain unchanged as a result of the compensatory recipe adjustments I did.

It is possible that this change in expansion might not be enough. There is still room for more movement. The obvious target is the flux MgO, it has the lowest expansion by far. I am going to remove some of it in favor of CaO, this should have a greater effect on overall glaze expansion than the above change (note however that it is possible that the glaze depends partly on the magnesia (MgO) for its silky appearance, substituting CaO for MgO is more likely to preserve this than Na2O for MgO).

Following is a calculation where I have eliminated the MgO sourced by talc, and added CaO.

DETAIL PRINT - Matte White Glaze
MATERIAL               PARTS  WEIGHT   CaO*   MgO*   K2O*  Na2O*   ZrO2 Fe2O3*   TiO2  Al2O3   SiO2
WEIGHT OF EACH OXIDE                   56.1   40.3   94.2   62.0  123.2  160.0   79.7  102.0   60.1
---------------------------------------------------------------------------------------------------  MATERIAL
Expan OF EACH OXIDE                    0.15   0.03   0.33   0.39   0.02   0.13   0.14   0.06   0.04   Cost/kg
---------------------------------------------------------------------------------------------------  -------
 NEPHELINE SYENITE..   47.71  446.40   0.01   0.00   0.02   0.08                        0.11   0.48     0.33
 KAOLIN.............   10.75  258.14                                                    0.04   0.08     0.24
 FLINT..............   25.42   60.00                                                           0.42     0.19
 WHITING............   12.61  100.00   0.13                                                             0.12
 SUPERPAX...........    1.50  183.00                               0.01                        0.01     0.00
 BENTONITE..........    2.00  489.48   0.00   0.00   0.00                 0.00   0.00   0.00   0.02     0.00
---------------------------------------------------------------------------------------------------  -------
TOTAL                  99.99           0.13   0.00   0.02   0.08   0.01   0.00   0.00   0.16   1.02     0.25
UNITY FORMULA                          0.56   0.01   0.10   0.32   0.03   0.00   0.00   0.67   4.34
PER CENT BY WEIGHT                     7.98   0.10   2.44   5.08   1.09   0.08   0.00  17.22  66.01
Cost/kg  0.25
  Si:Al  6.51
 SiB:Al  6.51
  Expan  7.39

As you can see, the expansion has increased much more this time; possibly it was not necessary to remove all the talc. A line blend of this adjustment with the original recipe would determine some intermediate compromise mixture. A simple three-interval line blend is done by mixing the trial 75:25, 50:50 and 25:75 with the original. An easy way to do this is described in the section about altering glaze temperature.

I am going to try one more change.

I will go back to the original recipe again, leave the potash feldspar alone, remove the talc, and adjust the rest of the recipe ingredients to preserve the SiO2 : Al2O3 ratio.

DETAIL PRINT - Matte White Glaze
MATERIAL               PARTS  WEIGHT   CaO*   MgO*   K2O*  Na2O*   ZrO2 Fe2O3*   TiO2  Al2O3   SiO2
WEIGHT OF EACH OXIDE                   56.1   40.3   94.2   62.0  123.2  160.0   79.7  102.0   60.1
---------------------------------------------------------------------------------------------------  MATERIAL
Expan OF EACH OXIDE                    0.15   0.03   0.33   0.39   0.02   0.13   0.14   0.06   0.04   Cost/kg
---------------------------------------------------------------------------------------------------  -------
 CUSTER FELDSPAR....   49.75  617.10   0.00          0.05   0.02          0.00          0.08   0.57     0.00
 KAOLIN.............   16.50  258.14                                                    0.06   0.13     0.24
 FLINT..............   15.75   60.00                                                           0.26     0.19
 WHITING............   14.50  100.00   0.14                                                             0.12
 SUPERPAX...........    1.50  183.00                               0.01                        0.01     0.00
 BENTONITE..........    2.00  489.48   0.00   0.00   0.00                 0.00   0.00   0.00   0.02     0.00
---------------------------------------------------------------------------------------------------  -------
TOTAL                 100.00           0.15   0.00   0.05   0.02   0.01   0.00   0.00   0.15   0.99     0.09
UNITY FORMULA                          0.65   0.01   0.24   0.11   0.04   0.01   0.00   0.67   4.35
PER CENT BY WEIGHT                     9.07   0.05   5.56   1.64   1.11   0.22   0.00  16.97  65.37
Cost/kg  0.09
  Si:Al  6.54
 SiB:Al  6.54
  Expan  7.23

The expansion has moved from 7.0, for the original, to 7.23 with this modest change. Again, it is possible that the optimum recipe is a mixture of this and the original as determined by a line blend.

Notice that throughout this calculation process, I kept the total recipe amount at 100. Also, I rounded recipe amounts on each calculation to avoid accuracy overkill. Since the Superpax and bentonite are added for non-chemical purposes of melt opacity and slurry suspension, as a final step I recalculated the recipe total to 96.5 and restored the superpax to 1.5 and the bentonite to 2.0.

You probably want to know whether this really worked! Well, it performed perfectly! All three trials had a surface quality that is almost identical to the original. The first two high sodium tests made the glaze a little less opaque, requiring the addition of more Superpax. A little work with the three in production indicated the best one.

Dealing with Crazing

In actual practice, it is not that common to find a glaze that shivers and requires the kind of adjustment just done (except perhaps at low temperatures using high talc bodies). Middle and high temperature glazes naturally tend toward crazing, it seems it is always a challenge to keep the expansion low enough. The process is simply the opposite of that shown above, the best strategy is to reduce Na2O and K20 in favor of MgO and CaO or to add boron which enables increasing the silica and alumina.

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By Tony Hansen




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