|Monthly Tech-Tip |
In hobby ceramics and pottery it is common to layer glazes for visual effects. Using brush-on glazes it is easy. But how to do it with dipping glazes? Or apply brush-ons on to dipped base coats?
In hobby ceramics (at low temperatures) layering of glazes for decorative effects is commonplace, there are what seem like myriads of choices of bottled glaze products. Even stoneware potters are increasingly adopting bottled brush-on glazes for their stoneware pottery and porcelain, suppliers show rooms are now dominated by bottled glazes. The idea of mixing your own recipe and dipping ware now seems quaint to many potters. This is in spite of the fact that bottled glazes are very expensive. Typically, three coats are applied. Each dries slowly, hardening as it does so (the glazes contain binders). This provides a stable base for the next one. Although the process is slow (compared to dipping) and it is more difficult to get even coverage, the interactions between layers of varying degrees of opacity, melt fluidity and especially color can produce stunning visual effects.
As noted, commercial bottled glazes contain a binder, typically CMC gum. Why? Consider traditional dipping glazes. They are intended as single-layer. They are just powdered minerals with a percentage of clay, typically 15-25%, it suspends the slurry and hardens the layer as it dries. Dipping glazes can apply to bisque in the needed thickness with one quick dip and they can dry enough to permit handling within seconds. But, their bond with the bisque ware is fragile, especially if the surface is smoother or more dense or the bisque is damp. Still, as long as the layer is not too thick, the bond is able to withstand the tension imposed by the shrinkage during drying (without cracks forming). But, the traditional dipping glazes normally can only be layered under special conditions. If the bisque is absorbent, the first layer is still damp (not wet) and it is not too thick a second dipped coat may be tolerated. Likewise, control of the raw clay content can help make multi-layering possible (e.g. if a recipe contains 30% raw kaolin, 15% calcined and 15% raw would reduce the drying shrinkage, if a recipe lacks clay (and therefore is not bonding well with the bisque) sourcing some of the Al2O3 from kaolin instead of feldspar will bond it better (you need glaze chemistry to do that).
But for serious multi-layering of glazes that you mix yourself there is one principle method: Adding gum to the recipe. 1% is a good starting point. Gummed glazes are more runny, they drip (and drip more) and dry slowly on bisque. Traditional potters, used to single-layer dipping glazes, can be shocked at how different it is to work with gummed glazes. They won’t flocculate, the only practical way to make them thicker is raising the specific gravity (as high as 1.6, vs. dipping which can be as low as 1.4). You need to learn ways to apply them evenly and quickly. More absorbent bisque helps (e.g. fired to cone 06). Even heating thinner ware before dipping will greatly accelerate drying speed. Ware needs to be designed to be able to drain to a single point after dipping (since it is often necessary to hold a piece upside down for a minute while waiting for dripping to stop). Of course, spray application is also an option, but multilayering will likely still be necessary. And runs will be a constant issue.
Layering Commercial Bottled Glazes Over a Base Coat you Mix Yourself
As noted above, your glaze needs to be gummed (e.g. 1%). If it is then it will tolerate layers of commercial products. All ware needs some sort of cover or base glaze and it makes a lot of sense (especially economic sense) to mix that yourself. You may find that a low percentage of gum will work, that will be a benefit since the glaze will dry fast and go on more evenly.
Firing Issues Regarding Layering of Glazes
-If multiple layers all have high melt fluidity: Each layer will need to be thinner than normal. Or contours will need to be modified to be able to tolerate more running.
-A fluid first layer and a non-fluid second: Caution will still be needed as the weight of the second will pull downward on vertical walls.
-A non-fluid layer first and a fluid second layer: Here there is opportunity to make both of them thicker.
-Two non-fluid layers: Why would you do that? Layering is almost always about making them dance together, interact. And non-fluids will not do that.
-Three layers: Now you are getting adventurous!
Another factor to consider is thermal expansion. Layering a glaze that normally crazes with one that does not could cause cracking when ware exposed to sudden temperature change (because of the internal stresses that would be present).
Commercial glazes are labelled as being food safe, even if the color is very bright. Traditional potters are not accustomed to putting high-percentage-heavy-metal glazes on food surfaces. But that is what many commercial ones are. Use common sense. It is often better to employ non-coloured glazes on food surfaces that must accommodate hot or acidic liquids. When you make your own glazes for these surfaces you know the recipe and you are in control. Consider: Two different drugs may not have serious side effects on their own, but when taken together they can be dangerous. Likewise, a glaze may resist acid attack on its own but you can be sure there exist other glazes, that when mixed with or layered with it, will destabilize it enough to leach metals.
The base glaze (inside and out) is GA6-D Alberta Slip glaze fired at cone 6 on a buff stoneware. However on the outside the dried glaze was over-sprayed with a very thin layer of titanium. The dramatic effect is a real testament to the variegating power of TiO2. An advantage of this technique is the source: Titanium dioxide. It is a more consistent source of TiO2 than the often-troublesome rutile.
This bowl was dipped in a non-gummed clear dipping glaze. Such glazes are optimized for fast drying and even coverage. However their bond with the bisque is fragile. The blue over-glaze was applied thickly on the rim (so it would run downward during firing). But during drying, it shrunk and pulled the base coat away at the rim (likely forming many tiny cracks at the interface between the clear and the bisque. That initiated the cascade of crawling. When gummed dipping glazes are going to be painted over, a base-coat dipping glaze should be used. What is that? It is simply a regular fast-dry dipping glaze with some CMC gum added (perhaps half the amount as what would be used for painting). There is a cost to this: Longer drying times after dipping and less even coverage. And gum destroys the ability to gel the glaze and make the slurry thixotropic.
Non-gummed dipping glazes go on evenly and dry quickly on bisque ware (if properly gelled). But they only work well as a single layer. If you try to paint commercial gummed brushing glazes over them the latter will compromise their bond with the body, cracks will develop during drying and bare patches like this will result during firing. For multi-layering the base dipping glaze must be gummed (e.g. 1% CMC gum). It will go on thinner, drip longer and dry much slower, but that is the price to pay if you want to layer over it.
This is a clear glaze (G2931K) with 10% purple stain (Mason 6385). The mugs are cone 03 porcelain (Zero3). The mug on the left was dipped (at the bisque stage) into a slurry of the glaze (having an appropriate specific gravity and thixotropy). The glaze dried in seconds. The one on the right was painted on (two layers). Like any paint-on glaze, it contains 1% CMC Gum. Each layer required several minutes of application time and fifteen minutes of drying time.
These are cone 6 Alberta Slip recipes that have been brushed onto the outsides of these mugs (three coats). Recipes are GA6C Rutile Blue on the outside of the left mug, GA6F Alberta Slip Oatmeal on the outside of the center mug and GA6F Oatmeal over G2926B black on the outside of the right mug). One-pint jars were made using 500g of glaze powder, 75g of Laguna CMC gum solution (equivalent to 1 gram gum per 100 glaze powder) and 280g of water. Using a good mixer you can produce a silky smooth slurry of 1.6 specific gravity. However most commercial glazes do have a lower specific gravity (have more water), this does aid further in paintability but requires more coats. Amazingly, the presence of the gum also makes it unnecessary to calcine the Alberta Slip.
This is a low fire brushing glaze. It has been sitting on this plaster bat for two hours and shows little sign of dewatering. A typical pottery dipping glaze, by contrast, would dewater in seconds! Clearly, such glazes are only good for brushing.
Example of the variegation produced by layering a white glaze of stiffer melt (a matte) over a darker glaze of more fluid melt (a glossy). This was fired at cone 6. The body is a stoneware and the glazes employ calcium carbonate to encourage bubbling during melting, each bubble reveals the color and texture of the underlying glaze layer. It is also possible to get this effect using the same base glaze (stained different colors).
On the left is a pure blue stain, on the right a green one. Obviously, the green is much more refractory. On the other hand, the green just sits on the surface as a dry, unmelted layer. For this type of work, stains need to be mixed into a glaze-like recipe of compatible chemistry (a medium) to create a good, paintable color. The blue is powerful, it would only need to comprise 5-10% of the recipe total. Its medium would need to have a stiffer melt (so the cobalt fluxes it to the desired degree of melt fluidity). A higher percentage of the green stain is needed, perhaps double. It's medium needs much more melt fluidity since the stain is refractory. Of course, only repeated testing would get them just right. Guidelines of the stain manufacturer for chemistry compatibility need to be consulted also (as certain stains will not develop their color unless their glaze medium host has a compatible chemistry). And, to be as paintable as possible, use use a gum-solution/water mix (e.g. 2 parts water to one part gum solution).
An example of a highly fluid glaze melt that has pooled in the bottom of a bowl. The fluidity is partly a product of high KNaO, thus it is also crazed (because KNaO has a very high thermal expansion). While it may to decorative, this effect comes at a cost. The crazing weakens the piece, much more than you might think (200%+). Those cracks in that thick layer at the bottom are deep, they want to continue down into the body and will do so at the first opportunity (e.g. sudden temperature change, bump). Also, fluid glazes like these are more likely to leach.
The underglaze is G1214M cone 6 black (adds 5% Mason 6666 black stain). Overglaze left: GR6-H Ravenscrag Oatmeal. Overglaze right: GA6-F Alberta Slip oatmeal. Both produce a very pleasant silky matte texture (the right being the best). Both layers are fairly thin. In production it would be best to spray the second layer, keeping it as thin as possible. It is also necessary to adjust the ratio of raw to calcined Alberta or Ravenscrag Slips to establish a balance between drying hardness but not too much drying shrinkage (and resultant cracking).
The glaze on the right is crawling at the inside corner. Why? Multiple factors contribute. The angle between the wall and base is sharper. A thicker layer of glaze has collected there (the thicker it is the more power it has to impose a crack as it shrinks during drying). It also shrinks more during drying because it has a higher water content. But the leading cause: Its high raw clay content increases drying shrinkage. Calcining part of the raw clay destroys its affinity for water (which is what makes it plastic), this is an effective way to deal with this. Or doing a little chemistry to source some of the Al2O3 from materials other than clay (e.g. a frit having a higher Al2O3 content).
Base-Coat Dipping Glaze
These are ceramic glazes intended for dipping but which contain a gum to enable them to adhere to the body better and tolerate over-layers without danger of flaking or cracking.
Hobbyists and increasing numbers of potters use commercial paint-on glazes. It's convenient, there are lots of visual effects. There are issues compared to dipping glaze. You can make your own.
Calcining is simply firing a ceramic material to create a powder of new physical properties. Often it is done to kill the plasticity or burn away the hydrates, carbonates, sulfates of a clay or refractory material.
Thixotropy is a property of ceramic slurries. Thixotropic suspensions flow when you want them to and then gel after sitting for a few moments. This phenomenon is helpful in getting even, drip free coverage.
In ceramics, the specific gravity of casting slurries and glazes tells us their water-to-solids. Body slurries especially require tight control of this property for performance reasons.
A ceramic glaze fault that occurs during firing of the ware, islands of glaze form as it crawls, leaving bare patches of body.