Glaze CoTE Boric Oxide Project


Introduction and Purpose

This project was first suggested by Ron Roy. I had conversed with him about glazes over email. Ron Roy co-wrote one of the seminal books on glazes for cone six pottery, Mastering Cone 6 Glazes. I was to mix, construct, and fire the glaze rods to be tested, and Ron would do the invaluable step of grinding them down and measuring the thermal expansion in a dilatometer. What follows is that collaboration.


Explanation of glaze thermal expansion reality/computation can be simplified to the weighted sum of expansions of the various oxides the glaze is composed of when fired. A further simplification, assumed by ceramicists, is each oxide's contributed thermal expansion is linear throughout the proportions it can take in a glaze.


This project concerns where that assumption fails, specifically the case of higher percentages of lithium oxide and boric oxide. The thrust of this research is to investigate where thermal expansion changes from a negative to positive with changes in boric oxide content. There is some literature suggesting this occurs once Boron exceeds 14 to 16%.


The mechanism proposed in the literature, for this expansion reversal is a change from tetraborate to diborate ring formations in the boro-alumino-silicate glass matrix but confirmation of this is beyond the scope of this research.


Raw Materials

The glaze to be tested will be composed of raw materials, frits, and chemicals; the composition of which is listed below.

  • Boric Acid(Optibor TG ) Accuracy Confidence: High
    B2O356.5
    LOI43.5

  • Feldspar G-200 HP (Imerys G-200 HP can be found here) Accuracy Confidence: Med-High
    K2O13.2
    Na2O1.52
    CaO0.750
    Fe2O30.900
    Al2O318.2
    SiO2 65.9
    LOI0.160

  • Frit Ferro 3134 (DigitalFire's assay of F3134) Accuracy Confidence: High
    CaO20.10
    Na2O10.30
    B2O323.10
    SiO246.50

  • Whiting (Ground limestone/marble) Accuracy Confidence: Medium-high
    Based on the Huber Q-325 composition; the computed Whiting composition used is
    CaO54.14
    MgO0.96
    SiO21.0
    LOI43.9

  • EPK Kaolin (Edgars Minerals, Inc.) Accuracy Confidence: Med High
    SiO245.73
    CaO0.18
    Al2O337.36
    MgO0.098
    Fe2O30.79
    Na2O0.059
    TiO20.37
    K2O0.33
    P2O50.236
    LOI13.91

  • Talc (Texas Talc)    
    Amtal C-98 is what is used in this project. Below, Amtal CB 27:73 is a mixture of 73% uncalcined Amtal C-98 and 27% calcined Amtal C-98 (not sold). The percents of all three are listed below
    Amtal      C-98    Calcined    CB 27:73
    SiO2      54.3      60.76      56.0
    MgO     29.5     33.01     30.4
    CaO     3.5     3.92     3.6
    Al2O3     0.45     0.50     0.5
    Fe2O3     0.45     0.50     0.5
    TiO2     0.9     1.01     0.9
    K2O     0.27     0.30     0.3
    LOI     10.63     0.00     7.8

  • Silica (US Silica - SIL-CO-SIL 52) Accuracy Confidence: High
    SiO299.64
    CaO<0.01
    Al2O30.20
    MgO<0.01
    Fe2O30.026
    Na2O<0.01
    TiO20.02
    K2O0.01
    LOI0.01


Recipes

The base recipe is the Ron Roy/John Hesselberth "Mastering Cone 6 Glazes" Licorice glaze recipe without the colorants (Glossy Base Glaze 2, pg 96), with G200HP feldspar substituted for Custer, then the rest adjusted compensated for.


Rather than starting with the percentage of the ingredients used in Glossy Base Glaze 2 (GBG2), I targetted developing a recipe coming close to getting the same oxide unity composition. This circumvents problem of differing of ingredient compositions, seen between locations and mine lots. Getting the sum of alkali oxides (K2O, Na2O) close to the same and the sum of the alkali earth (CaO, MgO) the same was more important that each separate flux oxide the same.


OxideGBG2 GlazeTarget Glaze
CaO0.5480.547
MgO0.1470.148
K2O0.0940.114
Na2O0.2100.191
TiO20.0020.005
Al2O30.3860.390
B2O30.3350.342
P2O50.0000.001
SiO24.1204.120
Fe2O30.0040.004

These are the resulting glazes:

GlazeLicorice w/ 12% B2O3w/ 18% B2O3
Frit 313426.34%23.79%21.28%
G-200HP20.26%18.30%16.37%
Talc, C-984.96%4.48%4.01%
Whiting4.03%3.66%3.27%
EPK Kaolin17.12%15.47%13.83%
Silica27.25%24.62%22.01%
Boric Acid0.0%9.67%19.23%


The fired compositional breakdown for the Glossy Base Glaze 2:

w/ 12% Boric Oxidew/ 18% Boric Oxide
OxidePercentUnity
CaO7.750.547
MgO1.500.148
K2O2.710.114
Na2O3.000.191
TiO20.100.005
Al2O39.990.388
B2O312.000.682
P2O50.040.001
SiO262.714.128
Fe2O30.170.004
OxidePercentUnity
CaO7.230.547
MgO1.400.148
K2O2.530.114
Na2O2.790.191
TiO20.100.005
Al2O39.310.388
B2O318.001.098
P2O50.030.001
SiO258.434.128
Fe2O30.160.004


Endpoint Recipes

The base glaze (GBG2) is used to prepare the 12% and 18% recipes.


Target RecipeGBG2Boric AcidTotal
12%722.6 g77.4 g800 g

Target Recipe12%Boric AcidTotal
18%361.839.2400 g


Mixtures

The mixes creating the target 12%, 14%, 15%, 16%, and 18% using the 12% and 18% recipes as the base, with the following proportions:

Target Recipe12% portion18% portion
12%1 parts0 parts
14%2 parts1 parts
15%1 parts1 parts
16%1 parts2 parts
18%0 parts1 parts

Procedure

The glaze picked as the base for this test is Mastering Cone 6 Glazes: Glossy Base Glaze 2, then modified to use G-200HP instead of Custer Feldspar.


To the glaze, boric acid was added to increase the boric oxide content to 12 and 18%, respectively. The glaze is expected to be suspended in a non-aqueous, non-polar solution to prevent the boric acid from dissolving in the glaze.


The glaze base is to be mixed and sieved through an 80 mesh sieve, then amounts measured to compound the 12% and 18% batches. Once the two batches are made, three more batches can be prepared, 14, 15, and 16%, as 2:1, 1:1, 1:2, mixtures of the 12 and 18% batches. This allows for 12, 14, 15, 16, and 18% batches. The 15% batch is being run because of its closeness to the turning point in CoTE.


The five batches will be mixed with a non-aqueous, non-polar solvent and allowed to build up enough glaze to fill the glaze boats such that a 3/8 inch by 3/8 inch by 3 1/2 inch bar of glaze can be produced for each compositional mix.


The boats will be fired to cone 6, soaked at that temp for 20 minutes, then soaked at 50 degrees below cone 6 for 30 minutes, cooled to 300 degrees below cone 6, soaked for one hour, then cooled normal.


If the bars are not clear of bubbles or inclusions, they may need to be fired once again.


The labeled bars will then be sent to Ron Roy for measuring actual expansion for the bars using a dilatometer.



Steps:
1. Setup power tools to shape Insulating Fire Brick (IFP) into Boats to be used to fuse the glaze samples into bars.

2. Saw bricks into width/heights of at least 1.5 inch by 1.5 inch to form boat blanks.

3. Cut dado groove 3/8 inch wide by 1 inch deep into boat blanks.

4. File inside of groove to form slight V shape (to facilitate glaze bar removal).
5. Cut/file pieces of IFP to use as plugs for ends of the grooves in the boats.

6. Here are the boats formed from the IFP.

7. I mixed up some alumina hydrate and water, then painted the inside of the boats.

8. This is a closer view. The boats need three coats.

9. Measuring the glaze base ingredients.

10. The 12% and 18% mixtures.

11. All the mixed cases, 12, 14, 15, 16, and 18% B2O3.

12. Mixed the different glaze batches with a non-polar solvent (mineral spirits) then poured into the glaze boats. Cone pack visible on kiln shelf.

13. Glaze batches have been fired, probably up to about cone 6.5 (Was just at cone 6, then soaked for 15 minutes, which took it up to between 6 and 7. The cone 7 is just starting to bend. Glaze bars contain visible bubbles, may need to be refired.

14. This is an enlargement of the cone pack, more clearly showing the state of the cone 5, 6, and 7 cones.

15. The glaze was fired into bars. These bars were gotten out of the glaze boats. The bars were extremely brittle with several breaking. When the bars were sent to have their CoTE measured, all but the 18% Boron batch, which was too short to work in the dilatometer.

Another 18% Boron bar was fired but it broke too, when removing from the boat. I suspect that much Boron seriously weakens the glaze. That bar has been mailed and analyzed.


16. This is a pic of the glaze bar ground down and ready to go into the dilatometer. This was on top of the worksheet Ron used for the 12% test.

Results

The data and results are in the form of dilatometer graphs. These graphs seem to show little difference in the Coefficient of Linear Thermal Expansion as the concentration of boric oxide proceeds from 12% to 18%.










Analysis and Conclusions

The thermal expansions were gotten for the following Boron formulations: 14%, 15%, 16%, and 18%. They are derived from the fairly linear portion of the graphs in the range of 20 to 540 degrees Celcius.



Formulation% change expansion / Deg C
14%0.0056391
15%0.0056074
16%0.0055970
18%0.0057471




From the above graph, the thermal expansion decreases as the Boron content increases, as expected, until 16% Boron content. Between 16 and 18% Boron, the change in thermal expansion reverses direction.


Determination of the CoTE was from the grainy images of the dilatometer graph output, therefore was not as precise as desirable.


The initial results indicate a reversal of CoTE occurred, with increasing Boron. That said, a single set of tests are a poor basis for deriving conclusions.


Another set of tests would increase the confidence of the conclusions. A good suggestion would be to use a different procedure for the incorporation of high levels of Boron. Use of a differing approach would help rule out any undiscovered errors inherent to this method.