This is an overview of the various mechanisms you can employ to make glazes dance with color, crystals, highlights, speckles, rivulets, etc.
While industry often avoids so-called 'reactive glazes' potters do not like the 'porcelain sink look' of typical industrial glazes. As a general rule the latter are more practical because they fire consistently while the former are typically temperamental and difficult. Potters who work in reduction fired high stoneware come by glazes with interesting surfaces quite easily (although often inconsistent). This is mainly because the raw natural materials they employ melt easily at these temperatures and their unprocessed nature (with particles of widely varying size, chemistry and mineralogy) creates a melt concoction that solidifies into surfaces having all sorts of crystal, rivulet, speckle and color variations for light to dance on.
But can such glazes be made for lower temperatures in oxidation? Yes, and potentially better because there is a magic ingredient available there that is almost never used by reduction firing potters (more on that in a moment). However, while blindly mixing hundreds of recipes from books and web pages might scare up a good high fire glaze or two, at middle temperatures there are things to worry more about (e.g. Crazing, leaching) and much fewer materials melt well. Knowing the 'mechanisms' of the various kinds of variegation is the best way to achieve, enhance and control it. Another way of putting it: 'understanding' these mechanisms opens up more doors than the lower temperature conditions close.
Consider an example of the vase on the cover of the mastering cone six glazes book by Ron Roy and John Hesselberth. This glaze exhibits a number of different kinds of variegation (e.g. crystallization, rivulets, speckling, opacity variation). Look closer and you will see the reason this glaze 'dances': melt fluidity. Notice how it runs down to the shoulder. All kinds of things happen in glazes that melt this much. If you compare the chemistry of this glaze with a typical cone 6 transparent the difference becomes obvious: much higher boron, it melts big time at cone six. Add some rutile and iron to a fluid base like this and all kinds of things happen. As you can see, variegation in glazes does not have to be a mystery, it happens for some understandable reasons.
To review, an important part of understanding reactive glazes is being familiar with the term 'fluidity'. A fluid glaze is one that melts more than normal to form a fluid molten liquid. Like their high feldspar high temperature counterparts, lower firing fluid glazes like to run off the ware. However at lower temperatures highly fluid glazes dance for another price: crazing. This is because typical fluxes have high expansion and low expansion silica and alumina are less plentiful. However this price does not necessarily have to be paid. Why? Through the magic of boron. Boron is an oxide contributed by most frits and gerstley borate, colemanite and ulexite. The magic is that it melts like a super-flux yet forms a glass like silica and has a really low thermal expansion.
Try looking very closely at a heavily variegated glaze and you will be able to formulate some theories on the way in which these visual effects could have formed during melting and freezing. Consider some of the ways you can provide conditions for these effects to grow and ways in which you can physically create them.
Varying the thickness of a transparent (or partly opacified) colored glaze will vary the intensity of color with depth (especially where the layer thins on the edges of sharp contours). Thickness variations can be achieved by pouring, double-dipping, brushing, waxing, and incising techniques.
I am referring here to normal stoneware glazes and use the term 'fluid' in that context. The extreme, to which I am not referring, is crystalline glazes, they have almost no alumina and are so fluid that a catcher is needed to capture the runoff. These can grow dazzling single crystals of huge size, but alas, they are impractical for functional ware (they craze like mad, are soluble, inconsistent, soft and very difficult to apply and fire) so let us return to the real world of stoneware glazes. Fluid glazes like to form micro crystals on the surface during cooling (low alumina, high flux). TiO2 materials like titanium dioxide and rutile seed crystal networks and encourage their growth (of course they do alter the color also, both because of their interaction with other oxides and, in the case of rutile, because of the presence of coloring oxides). A thin rutile wash applied to a glaze surface can even act as a crystal growth catalyst. High calcium and boron levels encourage the formation of calcium-borate crystals, high zinc stoneware glazes also crystallize when they are fluid. The addition of up to 4% tin in such glazes can magnify the effect. Slow cooling greatly enhances crystal growth. Small amounts of lithium (e.g. 1%) can have a remarkable variegating effect on rutile glazes, especially when colorants like iron are present. It is worth mentioning that unwanted crystal growth on glaze surfaces is termed "devitrification", typically it is seen as bad by industry.
Certain colorants (like iron, copper, manganese) and stains can be added in higher than normal amounts to fluid glazes to produce a completely saturated and metallic fired surface. This phenomenon is akin to the precipitation of sugar crystals while cooling a sugar saturate water solution. In this example, 8% iron has been used. Note that such glazes are obviously not for food surfaces.
You can add a coloring oxide that contains particulate matter that speckles the glaze surface. Manganese granular, illmenite, and granular rutile are examples. However these materials are heavy and tend to settle in glazes that are too fluid. Coarser grades of iron oxide and cobalt oxide often produce small specks in unmilled glazes. Using pure metal powder or filings is also also an option.
Layered glazes interact with each other chemically and mechanically. Two molten liquids will obviously diffuse into each other during melting. This diffusion does not occur evenly however since one glaze will be more fluid than the other; surfaces will be vertical, horizontal or in between; bubbles from body and glaze decomposition are rising through the melt stiring things up; the fluidity of the boundary zone will be variable, etc. In addition the changes in chemistry of the boundary zone will bring about changes in surface tension, fluidity, ability to dissolve unmelted particles, etc. Thus the result is a variegated visual appearance.
Double layering of different glazes produces variegation well when the lower layer is more fluid and the upper is stiffer (it will tend to break into islands revealing rivulets of the lower one). Or a much more subtle effect would be to put a thin layer of matte white over a glossy white, the effect will be something that no single-layer glaze could produce. Similarly, a contrasting colored fluid glaze over a stable one often has its own variegation surprises.
However be aware of the problems associated with double layer glazing (crawling during firing because of drying-induced cracks in the double layer during drying, these happen because the upper layer tends to compromise the body bond of the lower one when it shrinks during drying, especially if applied too thick or onto wet ware). Use glazes with lower or less plastic clay content for multilayer work, heat the ware before glazing so the glaze dries fast or bisque the first layer on.
Another mechanism of double layering is the use of a lower layer that releases gases during the melting phase. During melting the lower one will percolate up through the top layer creating molten craters that later heal leaving the visual after-effects of the process.
The glass matrix in a fired glaze can separate (or fail to mix) during melting forming globules of different glass chemistry. These reflect and refract light differently and thus variegate the visual characteristics of the surface, especially where colorants that react differently to the different phases are present. As expected, the visual effects are greatest when the glaze is thicker. 'Techies' look for chemistries that encourage phase separation. However populations of particles that have widely varying melting characteristics will also encourage it (e.g. really fluid particles of frit or lithium in a mix of clay, silica, colorant, tin and rutile will supply a population of particles with little in common so they won't melt as a team, the result will be variegation).
Use combinations of the above to variegate surfaces even more. The popular Floating Blue cone 6 recipe is a good example. Its color varies with thickness so it highlights irregularities in the surface. Phase separation in the translucent matrix makes the color 'swirl' in patterns of blue. Titanium crystals in the matrix make it sparkle. The growth of calcium-borate crystals on the surface appear to float over a deep blue background. However, while this is a popular glaze among potters it is understandably temperamental!
Some glazes greatly amplify thickness differences in a way that appears related to a number of the above factors. In these there is a dramatic change in color and character with thickness differences. The classic Albany:Tin:Lithium glaze is a good example. The effect is related to a large extent to a non-opacifying reaction between the tin and iron that needs a certain thickness to manifest itself fully. This glaze also works well using boron as a flux (instead of lithium).
An often overlooked method of creating variegation is actual physical intervention.
Knowing how to variegate using some of these techniques may not get you a job in a porcelain factory, but it will get the admiration of people who see your work. Variegation is typically a microsurface and glass structure chaos, controlling it can sometimes be like controlling chaos.
The referred to surface is the outside of this large bowl. The base glaze (inside and out) is GA6-D Alberta Slip glaze fired at cone 6 on a buff stoneware. The thinness of the rutile needs to be controlled carefully, the only practical method to apply it is by spraying. The dramatical effect is a real testament to the variegating power of TiO2. An advantage of this technique is the source: Titanium dioxide instead of sourcing TiO2 from the often troublesome rutile.
The mug on the left has been cooled slowly (and crystallizes). On the right it was cooled quickly (and is glossy).
A matte white glaze over a fluid dark colored glaze at cone 6
These glazes are both 80% Alberta Slip, but the one on the right employs 20% Ferro Frit 3249 accelerate the melting (whereas the left one has 20% Frit 3134). Even though Frit 3249 is higher in boron and should melt better, its high MgO stiffens the glaze melt denying the mobility needed for the crystal growth.
This is Ravenscrag Slip Oatmeal over a 5% Mason 6666 stained glossy clear at cone 6. You have to be careful not to get the overglaze on too thick, I did a complete dip using dipping tongs, maybe 2 seconds. Have to get it thinner so a quick upside-down plunge glazing only the outside is the the best way I think. You may have to use a calcined:raw mix of Ravenscrag for this double layer effect to work without cracking on drying.
This is an example of crystallization in a high MgO matte. MgO normally stiffens the glaze melt forming non-crystal mattes but at cone 10R many cool things happen with metal oxides, even at low percentages. Dolomite and talc are the key MgO sources.
G1214M Cone 5-7 20x5 Glossy Base Glaze
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.
Concentrate on One Good Glaze
It is better to understand and have control of one good base glaze than be at the mercy of dozens of imported recipes that do not work. There is a lot more to being a good glaze than fired appearance.
G1916M Cone 06-04 Base Glaze
This is a frit based boron base glaze that is easily adjustable in thermal expansion, a good base for color and a starting point to go on to more specialized glazes.
G1947U/G2571A Cone 10/10R Base Matte/Glossy Glazes
These starting recipes use no frits and work in oxidation/reduction and are inexpensive to make. They can be used as bases for the whole range of typical cone 10 pottery glazes (celadon, tenmoku, oatmeal, white matte, brown crystal).
Reducing the Firing Temperature of a Glaze From Cone 10 to 6
Moving a cone 10 high temperature glaze down to cone 5-6 can require major surgery on the recipe or the transplantation of the color and surface mechanisms into a similar cone 6 base glaze.
Stop! Think! Do not get addicted to the trafficking in online glaze recipes. Learn how they work. Understand them. Then make your own or adjust/adapt what you find online.
In ceramics, reactive glazes have variegated surfaces that are a product of more melt fluidity and the presence of opacifiers, crystallizers and phase changers.