We have moved our education section

You browser should take you to the new education home page now.
If it does not please Click Here

Home Products Services Education News
Clay | Glaze | Firing | Toxicity | Materials | Books | People | Events | Glossary

G1214M Cone 5-7 Glossy Base Glaze

Valuable Properties I Shoot For:

• I want a reliable glaze, I don't want to waste my time on 'cheap materials'.
• I want the glaze to work with as many stains as possible (so I don't want zinc because it has a detrimental effect on some colors and I can add it if needed for certain ones. I want minimal MgO so that chrome-tin pinks.
• I want a glaze that is not leachable so I don't poison or worry my customers.
• I want a glaze that applies well and evenly without drips or cracking even on very dense bisque ware.
• I want a hard fired surface.
• I don't want a touchy glaze that does not work if it is not exactly the right thickness or fired exactly the right temperature.
• I want a low expansion so it not only does not craze but strengthens ware that it is applied to.
• I want good transparency not only for its own use but as a clean starting point for other glazes I will make from it.
• I want to use materials that are easy to get.
• I want the glaze to be adjustable, I want to understand why each ingredient is there and why it is there in the amount it is.
• I want control, I want to understand the glaze's chemistry and be able to relate its oxide formula to what it does in the kiln.

The Starting Oxidation Recipe

There is a lot of merit in having a base gloss recipe that satisfies as many of the above requirements as possible and is adjustable. Adjustments can be rationalized in terms of this base glaze recipe and formula. A gloss glaze can even be a base for a matte version.

20% Kaolin

The glaze should have at 20% kaolin if possible to impart good suspension and application properties. I prefer EPK because it flocculates and gels the glaze so it applies without drips, even on dense bisque ware. I do not use ball clay because it contains iron that muddies stain colors, some of which are quite sensitive.

20% Silica

The recipe should have plenty of silica to minimize expansion and produce less crazing. It should have the ability to accept more if the expansion needs to be moved down.

20% Wollastonite

I want to be able to adjust the recipe to make a matte. Calcium mattes based on balanced formulations usually have a threshold amount of CaO (about 0.9) below which the glaze is glossy, above which it suddenly becomes matte. I need at least three times as much CaO as B2O3 for chrome-tin colors also (see below). CaO also has a low expansion compared to sodium and potassium. Thus I would like to have about 0.7-0.8 molar equivalents of CaO and I'd like to have it from a clean reliable source like Wollastonite. It is much better than whiting because it contains additional and beneficial finely dispersed SiO2, it helps to seed crystals when they are wanted, and it has no LOI to create extra bubbles in the glaze (of every 100 grams of whiting in your glaze, 45 go up the chimney!). To get 0.8 CaO we need 20% wollastonite.

20% Frit 3134

I want to use some B2O3 to induce melting and reduce thermal expansion. According to some limit formulas, up to 0.4 B2O3 is normal, this base recipe has only 0.2 (I don't want too much B2O3 because the more there is the less durable the glaze). Increasing B2O3 to 0.3 molar equivalent in this glaze will produce a much more fluid melt (you can go higher if there is adequate alumina to stiffen the melt but this comes at a cost, see below). Thus if the glaze is crazing I can add B2O3, then the more fluid melt will accept more flint and kaolin. SiO2, B2O3, and Al2O3 are all very low expansion oxides so this strategy can be used to take the expansion much lower. However remember that if B2O3 is above about 10% it can begin to increase glaze expansion (here it is only 5% so there is plenty of room to move).

Now what material should I use to source B2O3? Gerstley borate is partially soluble and gives problems with gelling and flocculation and even the manufacturer admits its inconsistent nature. A low alumina frit like Ferro 3134 is perfect because it means we can supply alumina with more than the usual kaolin to produce a better slurry.

For chrome tin pinks, red, maroons, etc. to work, B2O3 should be minimal and whatever is present should be balanced by at least 3 times as much CaO. This glaze has 0.2 B2O3 and 0.8 CaO, a 4:1 ratio so there is plenty of room to increase B2O3 if needed. It could still be raised to 0.25 and probably be OK.

Finally, a down side to boron in certain glazes is that it combines with silica to form borosilicate crystals which can make an transparent glaze go somewhat cloudy. See below for more information on this.

20% Potash Feldspar

I would like to have at least some K2O and Na2O to not only diversify the fluxes but impart other beneficial properties to the glaze (i.e. brightness in colored glazes). These oxides are supplied mainly by feldspars, and it just so happens that 20% feldspar (I used Custer feldspar at first, but soda feldspar worked well also) will round out the glaze nicely at about 0.8 CaO and 0.2 KNaO. Feldspar can be cut to make room for more frit if melt fluidity is not adequate.

Starting recipe with formula and analysis.

G1214M-CONE 6 CLEAR BASE GLAZE
==============================
RECIPE              AMT  OXIDE  FORM  ANAL
------------------------------------------
 WOLLASTONITE     20.00  CaO    0.79 14.84
 FRIT 3134        20.00  K2O    0.07  2.13
 KAOLIN           20.00  MgO    0.01  0.09
 FLINT            20.00  Na2O   0.13  2.78
 CUSTER FELDSPAR  20.00  TiO2   0.01  0.31
                         B2O3   0.21  4.79
                         Al2O3  0.34 11.74
                         SiO2   3.14 63.08
 EXPANSION:        7.15  Fe2O3  0.00  0.23
 WEIGHT:         298.90  MnO    0.00  0.03
                         L.O.I        3.49

For a web page showing this recipe with Hyperlinked materials and oxides (i.e. if you need to find substitute materials in your area), please visit http://digitalfire.com/recipes. To research materials in general visit http://digitalfire.com/material, to research oxides visit http://digitalfire.com/oxide.

This recipe might appear 'thrown together' but not so. It is a compromise that achieves a measure of all the desired properties listed above while having a conservative formula (it falls within typical limits except for the CaO which is a little high for the reasons explained). But most important, this recipe is a 'frame-of-reference' we can use to create other glazes with specific properties.

A New Start

With simple ceramic calculations you can adjust almost any property including expansion, gloss, surface texture, flow, and melting temperature. Furthermore, you can adjust this base glaze to produce almost any visual effect (remember of course that whiter bodies give brighter colors with stains, iron bodies subdue and alter the effect of most colorants). For starters:

• Add 10-15% superpax for an excellent white, especially if a dark body needs to be covered.
• Add 4% cobalt oxide for a very deep Arabia blue.
• Use 1.5% cobalt carbonate and 1% copper carbonate for a bright medium blue.
• Use 1.3% cobalt and 0.8% chrome for deep blue green, vary to adjust shade toward green or blue.
• Use 0.2% cobalt oxide and 5% rutile for a light variegated blue.
• Use about 7% copper carbonate for a vivid metallic green (soluble).
• Add 1-3% lithium to variegate and flux to add exciting effects (i.e. with cobalt, copper, iron). One user adds 3% lithium, 1% cobalt carbonate and 0.5% nickel in one variation and and 3% lithium, 2% copper carbonate and 1% iron in another. This was tested for heavy metal leachate by Kirby Labs in PA, USA and interpreted by a toxicologist who determined that the results were well below limits and thus safe for functional use.
• To make this glaze into a matte, simply calculate to raise the alumina to 0.5, lower the silica to 2.5, increase the CaO to 0.9 (you need to use ceramic calculations to do this i.e. INSIGHT).

Boron-Blue Clouding

This recipe is treading a fine line: As noted, it has plenty of CaO to make it easy to convert to a matte, work with pink stains, and encourage a lower expansion. However this makes it susceptible to reacting with the boron to form borosilicate crystals that make the transparent glass cloudy. Here are some factors to deal with this problem:

• Al2O3 has a stiffening effect on the melt and helps prevent the crystals from growing. Use INSIGHT to hold or drop the SiO2 while increasing the Al2O3 until the crystals are inhibited. The more alumina you can add the harder the fired glaze will be (this glaze can take more alumina than you might think without going matte, especially if the B2O3 is increased a little also to retain a good melt). An example of this strategy is our G1214Q recipe where we increased the Al2O3 to 0.45 and the B2O3 to 0.34. We also incorporated a cleaner soda feldspar which increased sodium and thereby reduced the CaO to 0.7. The result is an ultraclear recipe that shows some promise but does violate that 3:1 CaO:B2O3 requirement.

1214Q		1214Q Oxides	1214M Oxides
WOLLASTONITE	10	CaO - 0.7	0.8
FRIT 3134	30	B2O3 - 0.34	0.2
KAOLIN	25	Al2O3 - 0.45	0.35
FLINT	15	SiO2 - 3.3	3.1
F-4 FELDSPAR	20

• CaO normally tops out around 0.6 for cone 6 glazes. However we have found that making up most of the remaining 0.4 parts of the unity formula with K2O or Na2O results in higher thermal expansions than we would like. If CaO is taken to 0.9 parts thermal expansion is lower. An example is our 1213T recipe. Even though it has reduced alumina and silica expansion is very low and it produces an excellent transparent ultraclear with only a little more boron. If this recipe proves to have adequate strength, hardness, and resistance to leaching it is an excellent starting point for even lower expansion blends because there is lots of room for adding more SiO2 and Al2O3.

1214T		1214T Oxides	1214M Oxides
WOLLASTONITE	30	CaO - 0.9	0.8
FRIT 3195	30	B2O3 - 0.27	0.2
EPK KAOLIN	20	Al2O3 - 0.31	0.35
FLINT	20	SiO2 - 2.6	3.1

We have taken this farther yet in 1214W.

As a demonstration of how far you can go in the pursuit of clarity consider the following recipe. It is an ultra-clear transparent used by many people but it has many problems (i.e. not enough CaO to work with chrome-tin, it employs unreliable and troublesome Gerstley Borate, it oversupplies B2O3 enough to call into question durability and acid resistance, it has inadequate kaolin, it has a higher expansion).

CLEAR 2617
 KONA F-4 FELDSPAR   46.00  CaO   0.464*
 GERSTLEY BORATE     30.00  MgO   0.002*
 EPK KAOLIN          13.00  K2O   0.106*
 FLINT               11.00  Na2O  0.427*
                  ========  Fe2O3 0.003
                    100.00  TiO2  0.002
  Si:Al 5.836               B2O3  1.000
 SiB:Al 7.486               Al2O3 0.606
  Expan 7.611               SiO2  3.537

To get the B2O3 this high you have to use Gerstley Borate, no frit will work. Our advice: Resist the pressure to use glazes like this.

A Note About Test Tiles

It is wise to augment your testing with glaze tiles that are much thicker and heavier than the standard flat thin upright ones. A comparison of the glaze on thin and thick tiles will give you a good indication of its reaction to faster and slower cooling cycles (the heavier ones will cool slower and allow you to see any tendency for the glaze to devitrify or crystallize). Be sure to double-dip part of the sample to see what effect this has on fired character. Finally, be sure to study fired tiles closely. Do not discard a bubbled or clouded sample without looking at it carefully and trying to understand what happened in terms of the oxide makeup. Often apparent failed glaze tests can give you valuable information on oxide limits, interactions, trigger points, etc.

Further Improvements

As noted above, the 20-by-5 recipe described on this page is a reference point from which all kinds of other adjustments can be rationalized. For an example of how you could improve the clarity visit the web page at http://digitalfire.com/education/glaze/g1214w.htm. To reduce the thermal expansion consider the addition of a little lithium carbonate. Our 1215H introduces Li2O at the expense of high expansion Na2O and K2O and looks like this: Wolly 17, Frit 3134 18, EPK 21, Silica 24, Custer Feldspar 17, Lithium Carbonate 3. This change, although modest, reduces the thermal expansion considerably. To reduce the K2O and Na2O even further for a dramatically lower thermal expansion try G1215G which is EPK 20, Silica 30, Lithium Carbonate 3, Frit 3124 31, Whiting 14. Although this recipe appears quite different from the 5-equal-parts-recipe it is a direct derivative (it employs a different alumina-containing frit to prevent excessive shrinkage associated with high kaolin; and it uses more readily melting whiting to supply CaO).

Matte Version

Yes, we have a very good matte version of this glaze that can be tuned to fire with a surface very close to the classic dolomite magnesia mattes of cone 10 reduction. Visit the page at http://digitalfire.com/education/glaze/g1214z.htm.

Conclusion

Still can't get those textbook recipes out of your system? Try this. Any time you see an interesting glaze recipe in a book, determine what materials in its recipe cause the effect you want and do a calculation to see if the overall chemistry of the glaze is also involved. Then add them in the same amount to your own base glaze rather than the author's and adjust the base if needed to accommodate special additions. Stay in control as much as you can.

If you want to learn a lot more about glazes then check out our Magic of Fire book. It is on-line and available for download as an Acrobat document or coil-bound via airmail. If you want power to create and change glazes you also need INSIGHT software.

For an interesting article on cone 6 glazes by Val Cushing check this out:
http://www.studiopotter.org/articles/art0009.htm

Box 432, Cornwall, PE, C0A 1H0 Canada Phone Phone: (406) 662-0136

Top