|Monthly Tech-Tip |
This is a base transparent glaze recipe developed for cone 6. It is known as the 20x5 or 20 by 5 recipe. It is a simple 5 material at 20% each mix and it makes a good home base from which to rationalize adjustments.
The purpose of this page is not just to publish another recipe and throw you to the Glaze Dragon. For many the 'glaze recipe culture' and addiction to undocumented 'naked formulas' has meant countless 'blind alleys', years of wasted efforts, and gradual abdication of control to recipes that overstay their welcome and teach nothing. It has fostered a generation of ceramists with numbed consciences regarding their accountability for glazed ware they give or sell to others. The purpose here is to give you a 'starting point' so that you can exercise a degree of control over a base glaze to vary its color, surface, expansion, variegation, melting temperature, etc. This article links to other recipes that grew from this one. Other also have done the same, for example Ron Roy developed this into Base 1 in the book Mastering Cone 6 Glazes. I also want to raise your expectations with regard to your glazes. We also have starting points for cone 10, 6 and 06 glazes. If you decide to just take a recipe from this page and try it without understanding it you will need the Potter's Prayer (see links) also.
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.
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.
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.
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 silica 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.
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.
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 SILICA 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
Below there is a link to this recipe in the recipe area of this site.
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.
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:
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:
|WOLLASTONITE||10||CaO - 0.7||0.8|
|FRIT 3134||30||B2O3 - 0.34||0.2|
|KAOLIN||25||Al2O3 - 0.45||0.35|
|SILICA||15||SiO2 - 3.3||3.1|
We have taken this farther yet in the 1214W base recipe.
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* SILICA 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.
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.
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 see the G1214W article referenced in the links. 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).
Yes, we have a good matte version of this glaze (G1214Z) that can be tuned to fire with a surface very close to the classic dolomite magnesia mattes of cone 10 reduction (see the cone 10 base recipe article link).
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.
For an interesting article on cone 6 glazes by Val Cushing check this out:
This demonstrates how the host glaze affects the color development of certain stains. Blue is stable in pretty well all glazes. But chrome tin pink (top row) is very particular that the glaze have the right chemistry (1214M is obviously best, it has the highest CaO and lowest B2O3). The 6100 brown works much better in the N and O base glazes (they have higher Al2O3). Stain companies have guidance on chemistry particulars and you can view the chemistry of your recipe in your account at insight-live.com.
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.
The cone 6 G1214M glaze on the left melts well. Can it benefit from a silica addition? Yes. The right adds 20% yet still melts as well, covers better, is more glossy, more resistant to leaching, harder and has a lower thermal expansion.
Bringing Out the Big Guns in Craze Control: MgO (G1215U)
MgO is the secret weapon of craze control. If your application can tolerate it you can create a cone 6 glaze of very low thermal expansion that is very resistant to crazing.
Where do I start in understanding glazes?
Break your addiction to online recipes that don't work or bottled expensive glazes. Learn why glazes fire as they do. Why each material is used. How to create perfect dipping and drying properties. Even some chemistry.
What is the Glaze Dragon?
At Digitalfire we use a Dragon to personify the kinds of thinking that prevent potters, educators and technicians from understanding and therefore controlling their ceramic glazes.
Fighting the Glaze Dragon
At Digitalfire we promote the idea of understanding and formulating your own glazes so you have control rather than relying on suppliers or the trade in glaze recipes.
The Potter's Prayer
A prayer for potters who wish to continue down the road of text book glaze recipes, never really getting what they want, never getting control.
G1214W Cone 6 transparent glaze
The process we used to improve the 20x5 base cone 6 glaze recipe
G1214Z Cone 6 matte glaze
This glaze was developed using the 1214W glossy as a starting point. This article overviews the types of matte glazes and rationalizes the method used to make this one.
G1916M Cone 06-04 transparent glaze
This is a frit based boron glaze that is easily adjustable in thermal expansion, a good base for color and a starting point to go on to more specialized glazes.
G1215U - Low Expansion Glossy Clear Cone 6
A recipe sourcing high MgO (from Ferro Frit 3249) to produce a low expansion glass resistant to crazing on lower silica porcelains.
G1216M - Cone 6 Ultraclear Glaze for Porcelains
Substitute for low expansion cone 6 G1215U, this sources MgO from talc instead of a frit
G1214Z1 - Cone 6 Silky Matte
This glaze was born as a demonstration of how to use chemistry to convert a glossy cone 6 glaze into a matte.
G1214M - Original Cone 6 Base Glossy Glaze
A recipe developed by Tony Hansen in the 1980s. Its was popular because of the simplicity of the recipe and how well it worked with chrome-tin stains.
G1214W - Cone 6 Transparent Base
A cone 6 base clear glaze recipe developed by deriving a recipe from a formula taken as an average of limit formulas
G1216L - Transparent for Cone 6 Porcelains
Incorporates some MgO (at the expense of CaO, KNaO) to reduce the thermal expansion of G1214M 5x20 glaze.
Medium Temperature Glaze
These are stoneware glazes that fire in the range of 1200C (2200F). They often contain boron to assist with melting.
|By Tony Hansen|
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