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The Effect of Glaze Fit on Fired Ware Strength

Section: Glazes, Subsection: Thermal Expansion


The fit between body and glaze is like a marriage, if is is strong the marriage can survive problems. Likewise ceramic ware with well fitting glaze is much stronger than you think it might be, and vice versa.

Article Text

Shotgun marriages are not likely to last. Although this is true with people, I'm referring again to marriages between the glaze and clay on a fired piece of pottery. These two have to live together "for better and for worse". Most of us have always felt that "if they look good together" then everything is fine! However, continuing to ignore incompatibility won't make it go away. There will be a day of reckoning. Yes, I'm going to flog the subject of glaze fit one more time.

I have done tests that indicate it is possible for a good "marriage" of clay and glaze to double the strength of your unglazed ware. Conversely, a poor fitting glaze can cut it to one third of unglazed strength. That's one sixth of optimum! Incredibly, this range can occur without any visible difference in an otherwise attractive glaze, not even crazing. Are you using an attractive matte glaze like I was? Sometimes our ignorance can be such bliss when things look good! Many potters and technicians just do not want to know if there is a serious problem with their glaze. I am assuming you, however, would like to optimize things.

If the strength of the clay-glaze 'marriage' determines the strength of the entire piece, what can one do to get it off to a fine start, help them "become one" and stick together amidst trials and tribulations? Awareness is a good place to begin. Let's form a model to help us understand things better.

When clay objects are heated they expand; when cooled they contract. Since clay and glaze have to live attached to each other, they must expand and contract together. The degree of match between the two is referred to as "glaze-fit". A poor fitting glaze expands at a different rate than the clay to which it is attached. Usually there is no visible indication that there is a glaze fit problem, especially if the two are joined or interfaced well. But beneath that beautiful exterior, tension may threaten continued survival of the piece.

A glaze which is under some compression actually strengthens the ware. This is analogous to prestressed concrete where tension is maintained on steel reinforcing bars while concrete is poured and allowed to set. In my tests, I have noted an incredible fourfold variation in ware strength for a range of glaze expansions insufficient to cause even visible crazing or shivering. Thus, a visual examination is not enough to determine if a glaze is weakening or strengthening your ware, now or in the future.

For many of us, the quality of the union between our clay and glazes is almost accidental. If someone claimed they could double the strength of your ware by changing the glaze recipe slightly, you would likely be all ears (yet it seems that it is possible to quadruple it in worst case situations!). If you have problems with easily chipped or broken ware, delayed crazing, ware losing the "ring" it had after firing, or ovenware cracking during use, then here is a methodology that can help. It is time to end the "buck passing", blaming the clay, and stop at the real source of the problem, the potter or glaze technician, the one holding the shot gun in this wedding!

One cannot improve glaze fit, and therefore ware strength, unless he can test it. Industry uses complex and expensive equipment to measure the expansion of fired glaze and clay samples over a range of temperatures. Orton, for example, manufactures dilatometers for this purpose. However, this is not really an ideal option for most. Not only is the expense prohibitive, but potters and small companies lack the expertise to use the methods and information to their benefit; to adapt it to their own unique circumstances. There are other less technical methods (e.g. fired rings, suspended strips) but if you've tried any of them, like me, you are less than impressed with the practical benefits.

I have a better way. Since the main purpose of refining a clay-glaze union is to assure continued harmony and therefore, maintenance of maximum ware strength, why not test its integrity by measuring their fired strength? Really, it seems to me that such a test is the very best, most applicable, and meaningful measure of clay/glaze compatibility. Rather than telling us what should happen when they are used together, it tells us what does happen.

Strength testing machines are normally simple hydraulic devices. They are used by many industries producing structural products. Different types of tests are done with them, including those for elasticity, compressive, and tensile strength. The Modulus of Rupture (M.O.R.) is of special interest. It is based on the amount of force necessary to break a bar at its mid point.

Based on the principles outlined below, it is possible to make a very simple strength testing device from a metal frame with a pivoting load that presses on the center of the bar that straddles two supports at each end. The loading arm has an attachment for a standard torque wrench with an inert needle. Once the device is set up you simply insert the specimen to be broken, press down on the wrench to break it, and record the force necessary to do so. This figure goes into an equation to compensate for the specifics of your device, then into the equation outlined below to produce the strength figure.

Modulus of Rupture

According to the ASTM (American Society for Testing and Materials) Handbook, test C 674- 77: "Specimens, either cylindrical, or rectangular, are supported on knife edges over a suitable span and a direct load is applied at the midpoint between the supports at a uniform rate until breakage occurs".



As already mentioned, there are two ways to carry out this test: 'By the book' on a commercial strength testing device or on a homemade 'gizmo' you can afford.

The best test bars are round and extruded directly through a die of the correct cross sectional size. As a second choice, the test bars can be cut lengthwise from an undisturbed slug of clay. The slug should be pugged in a good pugmill, having a properly designed compressing die, and preferably a de-airing chamber. Other methods of making them can introduce laminations and differences that will throw the strength results out. The bars should be dried slowly to prevent drying stresses and then bisque fired. If you are using a home made device, it is obvious there will be compromises. Specimens will have to be smaller and more easily broken, and you will have to tolerate some variation until you can find a way to make specimens that break with consistent force. One real advantage of a strength tester of this type is that you can also use it to evaluate the dry strength of clay bodies to rate their plasticity.

Now, let's get to the tests I have been talking about. Another potter, Clara Matthews, and I, wanted to find ways of making ovenware resistant to breakdown from thermal cycling, so we worked together to test the above theories. We selected two glazes, a typical fatty matte that potters drool over and a clear glossy. We will consider the matte here; it is by far the most likely to weaken your pots. Two clay bodies, a porcelain and a buff stoneware, were chosen and 8 bars of each clay were dipped into the glaze. These 16, plus 8 unglazed bars, were fired to cone 10R. Finally, they were broken in a commercial strength testing device and the M.O.R. of each group of 8 was calculated and averaged. Results of this initial breakage showed the buff stoneware (H550) at 3930 psi with glaze and 3950 without; the porcelain (P580) at 3810 with the glaze and 9122 without! These figures turn on some red lights! Watch out for glazes of this type on porcelain. A forced clay-glaze 'marriage' here, without meaningful compatibility checking, will likely be a bad one. It also seems that a porous stoneware, having the right glaze, can be as strong as a porcelain with a poor glaze. There are more surprises to come! Small changes in a glaze can dramatically improve ware strength.

The next step involves dreaded calculation! There is really no other viable way. One warning: Calculations can easily be misused and misapplied. The computer makes them so easy that sometimes we fail to maintain proper balance between the direction indicated by calculated results, and that pointed to by physical observations. For example, many use a calculation approach that seeks to harmonize calculated thermal expansions for clay and glaze.

This method is invalid because fired expansion characteristics developed by oxide combinations are as much dependent on firing methods, particle size, glass development, etc., as they are on the formula. The former are not taken into account in calculations. Further, the calculated expansion of a body or glaze will never equal its measured expansion. Don't get me wrong, the key to solving this problem lies with calculation but not with this particular calculation approach.

The real challenge here is to alter the expansion of the matte glaze by changing its recipe but, if possible, doing so without altering fired appearance. Basically, you start by substituting fluxes of higher or lower expansion while maintaining the overall balance of the glaze. You can take more drastic measures, such as adjusting the SiO2 , B2O3 , or Al2O3 where appropriate.

I started with INSIGHT by entering the original recipe and producing a detail printed report of its formula, showing the expansion contribution of each oxide and the composite calculated expansion.

Calculating Initial Expansion

DETAIL PRINT - Matte White Glaze
MATERIAL               PARTS  WEIGHT   CaO*   MgO*   K2O*  Na2O* Fe2O3*   B2O3  Al2O3   SiO2
WEIGHT OF EACH OXIDE                   56.1   40.3   94.2   62.0  160.0   69.6  102.0   60.1
--------------------------------------------------------------------------------------------  MATERIAL
Expan OF EACH OXIDE                    0.15   0.03   0.33   0.39   0.13   0.03   0.06   0.04   Cost/kg
--------------------------------------------------------------------------------------------  -------
 CUSTER FELDSPAR....   41.00  617.10   0.00          0.04   0.02   0.00          0.07   0.47     0.00
 GERSTLEY BORATE....   12.00  133.20   0.04                 0.02          0.09                   3.19
 DOLOMITE...........    7.00  184.00   0.04   0.04                                               0.00
 KAOLIN.............    5.00  258.14                                             0.02   0.04     0.24
 TALC...............   15.00  126.23          0.12                                      0.16     0.25
 SILICA.............   20.00   60.00                                                    0.33     0.19
--------------------------------------------------------------------------------------------  -------
TOTAL                 100.00           0.08   0.16   0.04   0.04   0.00   0.09   0.09   1.00     0.47
UNITY FORMULA                          0.24   0.50   0.14   0.12   0.00   0.29   0.28   3.18
PER CENT BY WEIGHT                     4.60   6.82   4.46   2.54   0.11   6.76   9.73  64.97
Cost/kg  0.47
  Si:Al 11.33
 SiB:Al 12.35
  Expan  6.43

I then produced a glaze recipe of higher expansion by introducing high expansion fluxing oxides at the expense of ones of lower value. I did likewise to make a glaze of lower expansion.

I line blended the two new recipes to produce three intermediate mixtures, and thus a range of expansions. These five glazes were applied to more bisque bars and fired. Calculated M.O.R. figures revealed that further reduction in the expansion was necessary to increase strength to optimum. Later adjustments to restore matteness could then be made while maintaining the calculated expansion.

Next, I calculated another even lower expansion recipe and line blended with the original matte glaze, which now formed the upper expansion end. New bars were glazed, a firing done. You can see the results here.

Butter Matte Glaze C10 C9 C8 C7 C6
CUSTER FELDSPAR 41.00       41.00
CALCIUM BORATE 12.00       12.00
DOLOMITE 7.00       7.00
KAOLIN 22.00       5.00
TALC 9.00       15.00
SILICA 25.00       20.00
*CaO .28       .24
*MgO .41       .50
*K2O .16       .14
*Na2O .14       .12
*Fe2O3 .00       .00
B2O3 .33       .28
Al2O3 .58       .28
SiO2 4.31       3.18
EXPANSION 1.02       1.25
STRENGTH PSI Modulus of Rupture:
P580 9122 5597 4797 4137 4164 3811
H550 3955 8135 6538 5329 4887 3930

Notice that the C10 mix has doubled the strength of the H550 clay, a porous stoneware. Not only this, but a piece of pottery made from this combination would be stronger than a piece made from the porcelain using the same glaze! Incredibly, it would be twice as strong as a piece made from the porcelain using glaze C6, the original.

Sure, C6 looks great on the porcelain, it doesn't even craze. However, it produces a very weak piece of ware! C10, unfortunately, has lost some matteness. Further work using any number of calculation strategies will bring it back while maintaining the low expansion.

What have we learned?

Don't mix up these glazes and expect them to produce optimal strength on your clay. Really, for the very best ware, each clay-glaze combination should be tested. However, it is not really practical to do it this way for every glaze. After one or two series of tests, you can correlate calculated expansion values to the clays you use and calculate after that. It is best to stay away from the very high strengths that you might produce during testing (e.g. C10 on H550). This usually means too much compression, and over time, such pieces could weaken and fail.

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By Tony Hansen

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