All Glossary

Base Glaze

Understand your a glaze and learn how to adjust and improve it. Build others from that. We have bases for low, medium and high fire.

Key phrases linking here: base transparent, base recipes, cover glazes, base recipe, cover glaze, base glazes, base glaze, base matte - Learn more

Details

A base glaze is one having no opacifiers, variegators or colorants. Thus it should be transparent if glossy and translucent if matte. Developing or adapting a base glaze to your clay bodies and ware is a very important first step in creating good quality ware. In fact, it is much better to have a plain base transparent that is functional, durable and adjustable than to have multiple reactive, more visually interesting recipes that are not. The reason for this is that the additives in the latter can be transplanted into the former so that you can have it all!

To melt well enough low temperature glazes need a powerful flux and boron is almost universal. Base clear glazes for low temperatures normally contain high percentages of frit (up to 90%). And of course, at least some is clay to suspend it. Gerstley Borate has historically been used to source boron, but its days are numbered due to mine depletion and frustration with its physical properties. Boron has a low thermal expansion also, it enables much flexibility in adjusting the thermal expansion of glazes to prevent crazing.

At high temperatures, feldspars, calcium carbonate, dolomite, talc and other natural flux-oxide sources combine with clays and silica to melt very well as a mix (frits are normally not needed). Much higher percentages of silica and alumina are possible so. Unlike lower temperatures, it is less difficult to keep thermal expansion down, crazing is less of an issue. Lower percentages of fluxes (e.g. CaO, Na₂O, MgO) also makes it easier to produce base glazes that resist leaching and are more durable. However raw materials (as opposed to frits) produce more gases of decomposition, so it is more difficult to produce crystal clear glazes transparent as can be done with frits.

Middle temperature is where most potters do their work. More powerful fluxes are needed than for high fire, boron is the principle one used. Gerstley Borate appears very widely in recipes for this reason, but in much lower amounts than low-fire glazes (notwithstanding this cone 6 base clear glazes can have as much as 50%!). But again, frits are the best source of boron. Zinc, lithium and strontium are also employed as auxiliary fluxes. Middle-temperature base glazes, because they have less low expansion SiO₂ and Al₂O₃, can be prone to crazing, especially where high levels of feldspar are present (which is very common). It can be tricky to find a balance that produces a glaze of low enough expansion that still melts well, this underscores the importance of valuing a base recipe once you find one.

Once you achieve a stable, middle-of-the-road transparent base glaze then add an opacifier to get white. Then add stains for color. Of course, while everyone needs quality functional liner glazes, such bases do not easily produce the more interesting variegated surfaces. It is good to also have a fluid melt base, variegators and colors dance more within it (of course, experience is needed to learn how to apply such glazes so they do not run off ware onto the kiln shelves).

Once you have actual base recipe(s) that are working the next challenge is to turn them into a slurry that works well. It is good to be aware of the difference between dipping glazes, brushing glazes and first-coat dipping glazes, then mix them for what you need.

A well-documented testing program to understand how to adjust your base glaze will be one of the best investments in time you can make for your ceramic production. If you are a potter try to develop a mindset that the risk of losing ware in testing is worth the reward of better glazes.

Related Information

Glaze recipes online waiting for a victim to try them!

You found some recipes. Their photos looked great, you bought $500 of materials to try them, but none worked! Why? Consider these recipes. Many have 50+% feldspar/Cornwall/nepheline (with little dolomite or talc to counteract their high thermal expansion, they will craze). Many are high in Gerstley Borate (it will turn the slurry into a bucket of jelly, cause crawling). Others waste high percentages of expensive tin, lithium and cobalt in crappy base recipes. Metal carbonates in some encourage blistering. Some melt too much and run onto the kiln shelf. Some contain almost no clay (they will settle like a rock in the bucket). A better way? Find, or develop, fritted, stable base transparent glossy and matte base recipes that fit your body, have good slurry properties, resist leaching and cutlery marking. Identify the mechanisms (colorants, opacifiers and variegators) in a recipe you want to try and transplant these into your own base (or mix of bases). And use stains for color (instead of metal oxides).

A good base glaze, a vitreous clay and a good fit. How good that is!

Left: Cone 10R buff stoneware with a silky transparent Ravenscrag glaze. Right: Cone 6 Polar Ice translucent porcelain with G2916F transparent glaze. What do these two have in common? Much effort was put into building these two base glazes (to which colors, variegators, opacifiers can be added) so that they fire to a durable, non-marking surface and have good working properties during production. They also fit, each of these mugs survives a boil:ice water thermal shock without crazing (BWIW test). And the clays? These are vitreous and strong. So these pieces will survive many years of use.

In pursuit of a reactive cone 6 base that I can live with

These GLFL tests and GBMF tests for melt-flow compare 6 unconventionally fluxed glazes with a traditional cone 6 moderately boron fluxed (+soda/calcia/magnesia) base (far left Plainsman G2926B). The objective is to achieve higher melt fluidity for a more brilliant surface and for more reactive response with colorant and variegator additions (with awareness of downsides of this). Classified by most active fluxes they are:
G3814 - Moderate zinc, no boron
G2938 - High-soda+lithia+strontium
G3808 - High boron+soda (Gerstley Borate based)
G3808A - 3808 chemistry sourced from frits
G3813 - Boron+zinc+lithia
G3806B - Soda+zinc+strontium+boron (mixed oxide effect)
This series of tests was done to choose a recipe, that while more fluid, will have a minimum of the problems associated with such (e.g. crazing, blistering, low run volatility, susceptibility to leaching). As a final step the recipe will be adjusted as needed. We eventually evolved the G3806B, after many iterations settled on G3806E or G3806F as best for now.

Organized testing of stains in a base recipe

A quick and organized method of testing many different stains in a base glaze: Prepare your work area like this. Measure the water content of the base glaze as a percent (weight, dry it, weight it again: %=wet-dry/wet*100). Apply labels to the jars (bottom) showing the host glaze name, stain number and percentage added. Counterbalance a jar on the scale, fill it to the desired depth, note the amount of glaze and calculate the weight of dry powder that is present in the jar (from the above %). For each jar (bottom) multiply the percent of stain needed by the dry glaze weight / 100. Then weigh that and add to the jar and put the lid back on. Let them sit for a while, then shake and mix each (I use an Oster kitchen mixer). Then dip the samples, write the needed info on them and fire.

Mason stains in the G2926B base glaze at cone 6

This glaze, G2926B, is our main glossy base recipe. Stains are a much better choice for coloring it than raw metal oxides. Other than the great colors they produce here, there are a number of things worth noticing. Stains are potent colorants, the percentages needed are normally much less than metal oxides. Staining a transparent glaze produces a transparent color, it is more intense where the glaze layer is thicker, this is often desirable in highlighting contours and designs. If you add an opacifier, like Zircopax, the color will be less intense, producing a pastel shade the more you add. The chrome-tin maroon 6006 does not develop well in this base (alternatives are G2916F or G1214M). The 6020 manganese alumina pink is also not developing here (it is a body stain). Caution is required with inclusion stains (like #6021), the micro-bubbling here is not likely because it is overfired (it is rated to cone 8), adding 1-2% Zircopax normally fixes this issue.

Mason stains in the G2934 matte base glaze at cone 6

Stains can work surprisingly well in matte base glazes like G2934. But they perform differently in a matte host glaze. The glass is less transparent and so varying thicknesses do not produce as much variation in tint. Notice how low many of the stain percentages are here, yet most of the colors are still bright. A good reason to minimize stain concentration is to avoid leaching. We tested 6600, 6350, 6300, 6021 and 6404 overnight in lemon juice, they passed without any visible changes. It is known that MgO mattes, like this one, are less prone to acid attack that CaO mattes. A down-side to the MgO-matte-mechanism is that chrome-tin stains do not work (e.g. 6006), high CaO content is needed in the host glaze to develop the color. The inclusion stains 6021 and 6027 work very well in this base. As do the 6450 yellow and 6364 blue. And the 6600 produces an incredible gunmetal black. The 6385 is an error, it should be purple (that being said, do not use it, it is ugly in this base). The degree-of-matteness can be tuned by blending in some G2926B glossy base.

An ultra-clear brilliantly-glossy cone 6 clear base glaze? Yes!

I am comparing 6 well known cone 6 fluid melt base glazes and have found some surprising things. The top row are 10 gram GBMF test balls of each melted down onto a tile to demonstrate melt fluidity and bubble populations. Second, third, fourth rows show them on porcelain, buff, brown stonewares. The first column is a typical cone 6 boron-fluxed clear. The others add strontium, lithium and zinc or super-size the boron. They have more glassy smooth surfaces, less bubbles and would should give brilliant colors and reactive visual effects. The cost? They settle, crack, dust, gel, run during firing, craze or risk leaching. Out of this work came the G3806E and G3806F.

How do you turn a transparent glaze into a white?

Right: Ravenscrag GR6-A transparent base glaze. Left: It has been opacified (turned opaque) by adding 10% Zircopax. This opacification mechanism can be transplanted into almost any transparent glaze. It can also be employed in colored transparents, it will convert their coloration to a pastel shade, lightening it. Zircon works well in oxidation and reduction. Tin oxide is another opacifier, it is much more expensive and only works in oxidation firing.

These common Ferro frits have distinct uses in traditional ceramics

I used Veegum to form 10 gram GBMF test balls and fired them at cone 08 (1700F). Frits melt really well, they do have an LOI like raw materials. These contain boron (B₂O₃), it is a low expansion super-melter that raw materials don’t have. Frit 3124 (glossy) and 3195 (silky matte) are balanced-chemistry bases (just add 10-15% kaolin for a cone 04 glaze, or more silica+kaolin to go higher). Consider Frit 3110 a man-made low-Al₂O₃ super feldspar. Its high-sodium makes it high thermal expansion. It works really well in bodies and is great to make glazes that craze. The high-MgO Frit 3249 (made for the abrasives industry) has a very-low expansion, it is great for fixing crazing glazes. Frit 3134 is similar to 3124 but without Al₂O₃. Use it where the glaze does not need more Al₂O₃ (e.g. already has enough clay). It is no accident that these are used by potters in North America, they complement each other well (equivalents are made around the world by others). The Gerstley Borate is a natural source of boron (with issues frits do not have).

Brush-on commercial pottery glazes are perfect? Not quite!

Brushing glazes are great sometimes. But they would be even greater if the recipe was available. Then it would be possible to make it if they decide to discontinue the product. Or if your retailer does not have it. Or to make a dipping glaze version for all the times when that is the better way to apply. The glaze manufacturer did not consider glaze fit with your clay body, if they work well together it is by accident. But if you have transparent and matte base recipes that that work on your clay body then adding stains, variegators and opacifiers is easy. And making a brushing glaze version of any of them. Don't have base recipes??? Let's get started developing them with an account at insight-live.com (and the know-how you will find there)!

Two bases, 2% copper additions. Which is the better transparent?

Wrong. It is the one on the right. While the copper looks so much better in that fluid one on the left, that melt mobility comes at a cost: blisters. As a clear glaze it is no glossier than the other one, but it runs into thicker zones at the bottom and they blister. This is because the high mobility coupled with the surface tension blows bubbles as gases of decomposition travel through (in a normal cooling kiln they break low enough that mobility is insufficient to heal them). The fired glass in the one on the left is also not as hard, it will be more leachable, it will also craze more easily and be more susceptible to boron-blue devritrification. But as a green? Yes it is better.

What can you do using glaze chemistry? More than you think!

There is a direct relationship between the way ceramic glazes fire and their chemistry. These green panels in my Insight-live account compare two glaze recipes: A glossy and matte. Grasping their simple chemistry mechanisms is a first step to getting control of your glazes. To fixing problems like crazing, blistering, pinholing, settling, gelling, clouding, leaching, crawling, marking, scratching, powdering. To substituting frits or incorporating available, better or cheaper materials while maintaining the same chemistry. To adjusting melting temperature, gloss, surface character, color. And identifying weaknesses in glazes to avoid problems. And to creating and optimizing base glazes to work with difficult colors or stains and for special effects dependent on opacification, crystallization or variegation. And even to creating glazes from scratch and using your own native materials in the highest possible percentage.

Here is what happens if you do not sieve your glazes when needed

This is a cone 6 transparent base glaze. It contains frit, silica, kaolin, wollastonite. Almost all glazes have materials that are slightly soluble and over time these can form scale on the sides of the bucket or even precipitate particles into the slurry. The defects here are those scales. Before dipping a production piece in any glaze that has been in storage it is a good idea to assess it first to see if it needs to be sieved.

Commercial glazes on decorative surfaces, your own on food surfaces

These cone 6 porcelain mugs are hybrid. Three coats of a commercial glaze painted on the outside (Amaco PC-30) and my own liner glaze, G2926B, poured in and out on the inside. When commercial glazes (made by one company) fit a stoneware or porcelain (made by another company) it is by accident, neither company designed for the other! For inside food surfaces make or mix a liner glaze already proven to fit your clay body, one that sanity-checks well (as a dipping glaze or a brushing glaze). In your own recipes you can use quality materials that you know deliver no toxic compounds to the glass and that are proportioned to deliver a balanced chemistry. Read and watch our liner glazing step-by-step and liner glazing video for details on how to make glazes meet at the rim like this.

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