It can be difficult to find an engobe that is drying and firing compatible with your body. It is better to understand, formulate and tune your own slip to your own body, glaze and process.
Non-glaze slips (engobes) for pottery and tile decoration have long fostered great fascination. Almost everyone has marveled at the simple beauty of terra cotta ware decorated with white slip and finished with a transparent or white glaze. Many potters are adapting this age old process to stoneware and porcelain. Industry, especially tile, routinely applies slips and engobes (e.g. white engobes are put on darker burning dry tiles before applying glaze over top). Slips are almost universally used in the single fire process. This is logical since their key application is to cover over dark burning or dirty bodies made from local materials, in these cost conscious situations it makes little sense to fire more times than needed.
Pairing of an engobe with a specific body is difficult so slip and engobe recipes travel even less well than glazes. Slips need to be drying, firing and thermal expansion compatible with both the underlying body and overlying glaze. Each of these should be tested individually (I will show you how shortly, I am developing this article as of July 2014). Slips often contain silica (which is raw quartz) and can be sandwiched between a body and glaze that contain none, this is a recipe for trouble as the firing falls through quartz inversion. Compare this to a glaze being applied directly to a bisque body: you only need to focus on thermal expansion compatibility with the body; this is so much easier.
Consider the first major challenge with slips (or engobes): Compatibility and adhesion with the body (both in drying and firing), in other words getting he damn stuff to stick on! If the slip does not shrink at the correct rate or amount during drying and firing, cracks will develop or it will flake off. No matter what logic, theory or lab instruments might indicate or what others might advise, if cracking or flaking is occurring it is much more likely you need to react by adjusting the recipe of the slip appropriately than by adjusting the way it is prepared or applied. Different factors are involved in attaining compatibility for firing and drying and a change that improves one aspect of fitness may detrimentally affect another. While each material in the slip is there for a drying, firing, adhesion, thermal expansion or aesthetic reason, it is also potentially detrimental to one or more other parts of the process; this means developing the right recipe is a real juggling act.
The most obvious use of a fine-grained white engobe is to cover a dark colored (and possibly coarse-grained) body so that brightly colored or lightly shaded glazes appear as they do on fine porcelain. The tile industry is by far the largest user of engobes, it is very common for them to use red burning clays (they are often locally available and fire much stronger than light burning clays at low temperatures). They cover the red bodies with a white burning opaque engobe. Obviously good adherence to the fired body is paramount, so the engobe needs frit or other melters to create a glass bond. Adherence will obviously be best on vitrified bodies where an interface can develop (if the body does not vitrify well extra frit will be needed to create a more glassy interface, but not so much that opacity is lost).
Dry Adherence, Shrinkage, Hardness; Slurry Suspension
Fired Shrinkage and Adherence
The fired interface between slip and body will never be as good as one between glaze and body. This is thus potentially a great weakness if the fired adherence of the slip is not carefully evaluated and optimized. Since slips do not melt formulation is normally done on the recipe level (the physical properties must simply be observed, ceramic calculations are not really applicable).
Laydown and Application
You need a different mindset than with glazes to have success with engobes. Glazes smooth out when they melt, slips do not. Like paint, the surface you apply is the one you get after firing. Thus drips, variations in thickness, roughness, pinholes, bubbles, etc are going to show. Normally a glaze over-layer is not going to be thick enough to cover over problems in the slip laydown, infact, slip irregularities can produce glaze defects. Application for tiles is straightforward since the surface is flat and horizontal, but for uneven shapes it is a lot more complicated. Here are some ideas.
Conceptually a slip recipe can be as simple as a porcelain body with enough added frit for adherence but no so much that glass development reduces the opacity. This can be a fine line. For example, only 15% Ferro Frit 3134 may flux it too much at cone 04. It is better to mix your slip as a recipe of ingredients that include those for the porcelain rather than just adding something to a powdered porcelain whose recipe you do not know, this will give you the needed flexibility to adjust.
For high temperature you should be able to flux using feldspar and other raw materials rather than frits (e.g. wollastonite, dolomite). Thus, an alteration of the standard 25 porcelain could be the 40% kaolin, 35% feldspar, 25% silica (I have combined the ball clay and kaolin to just kaolin (for extra whiteness), you may need to add a plasticizer like Veegum. If it can be more mature, try 40 feldspar at the expense of silica this time. For medium temperature a typical base porcelain recipe might be 40 kaolin/ball clay, 40 feldspar and 20 silica. Add 5% of a boron frit (like Ferro 3134). The idea is to tune it degree of maturity to get maximum melting without loss of opacity.
For more whiteness consider using a whiter kaolin or adding some zircopax (perhaps 5%, for opacity). If the slip is too plastic, use calcined kaolin for part of the clay complement if needed to cut drying shrinkage. If it is flaking off, then try adding Veegum (up to 10%). And as a last resort use gum to harden the slip and give it better flow (however it is better to adjust or substitute clays to get the flow and drying properties needed, gummed slips dry slower).
Creating your own slip will take time, determination, and a methodical approach with plenty of adapting over time. Like glazes, it is far better to have one slip you understand and control than mess around with 10 that you don't understand and don't work?
An example of an engobe (slip) applied to dry ware and then fired at cone 2. The one of the left has been poured, the right sprayed. Control of the thickness of engobes is important, thus the specific gravity and viscosity of the slurries are very important. Engobes are invaluable since a red or brown burning body can be made to fire white like porcelain (enabling much better glaze surface quality).
Example of fritted white engobe over-fired to cone 2. During firing it has shrunk and the bond with the body has been completely compromised. The body has a firing shrinkage of about 6% but the slip is closer to 10%. Because glazes melt they do not have a firing shrinkage. But engobes do and it must be compatible with the body.
The white slip on the left is an adjustment to the popular Fish Sauce slip (L3685A: 8% Frit 3110 replaces 8% Pyrax to make it harder and fire-bond to the body better). The one on the right (L3685C with 15% frit) is becoming translucent, obviously it will have a higher firing shrinkage than the body (a common cause of shivering at lips and contour changes). The slip is basically a very plastic white body. Since these are not nearly as vitreous as red ones at low fire they need help to mature and a frit is the natural answer. With the right amount the fired shrinkage of body and slip can be matched and the slip will be opaque. This underscores the need to tune the maturity of an engobe to the body and temperature. Although zircon could be added to the one on the right to opacify and whiten it, that would not fix the mismatch in fired shrinkage between it and the body.
Slips and engobes are fool-proof, right? Just mix the recipe you found on the internet, or that someone else recommends, and you are good to go. Wrong! Low fire slips need to be compatible with the body in two principle ways: drying and firing. Terra cotta bodies have low shrinkage at cone 06-04 (but high at cone 02). The percentage of frit in the engobe determines its firing shrinkage at each of those temperatures. Too much and the engobe is stretched on, too little and it is under compression. The lower the frit the less the glass-bonding with the body and the more chance of flaking if they do fit well (either during the firing or after the customer stresses your product). The engobe also needs to shrink with the body during drying. How can you measure compatibility? Bi-body strips. First I prepare a plastic sample of the engobe. Then I roll 4 mm thick slabs of it and the body, lay them face-to-face and roll that down to 4 mm again. I cut 2.5x12 cm bars and dry and fire them. The curling indicates misfit. This engobe needs more plastic clay (so it dry-shrinks more) and less frit (to shrink less on firing).
This demonstrates the difficulty you can encounter when trying to get an engobe working with a clay body. Here the slip/glaze is flaking off the rim of pieces at cone 04 (does not happen at 06). The front bi-clay bar demonstrates the white and red clays dry well together (the slight curve happened on the drying). They also fire well together (the curvature did not change on firing). The back two thin bars seem to demonstrate thermal expansion compatibility: a thick layer of glaze is not under enough compression to curve either bar during firing. While the white clay contains 15% frit and forms a good bond with the red body, that bond is not nearly as good as the one between the glaze and the white slip. Yet it is still flaking off the rim at the slip/body interface. Why? At first it seemed that failure was happening at quartz inversion (because the body had less quartz than the white slip). However now it appears that the combination of compressions of the slip and glaze are sufficient to break the slip-body bond on convex contours. The compression of the slip and glaze likely did not demonstrate well on the bars because at this low a temperature they are not vitreous enough to be easily curled.
The white slip (applied to a leather hard cup) on the left is dripping downward from the rim (even though it was held upside down for a couple of minutes!). Yet that slurry was viscous with a 1.48 specific gravity, on mixer-off the motion stopped immediately. Why? Because it was not thixotropic (it did not gel). The fix? I watered it down to 1.46 (making it very thin and runny) and did a cycle of adding a pinch of epsom salts (about 0.5 gm) and mixing vigorously watching for it to thicken enough to stop motion in about 1 second on mixer shut-off (bounce backward!). It is extremely difficult not to overdo the epsom salts (gelling it too much) so I keep ungelled slurry aside and pour some back in to dilute to overgelled batch. That works perfect to fine-tune the degree of thixotropy so it gels after about 10-15 seconds of sitting. So to apply it I stir it, wait a couple of seconds and dip the mug. By the time I pull it out it is ready to gel and hold in place.
The engobe on the left, even though it has a fairly low water content, is running off the leather hard clay, dripping and drying slowly. The one on the right has been flocculated with epsom salts, giving it thixotropy (ability to gel when not in motion but flow when in motion). Now there are no drips, there are no thin or thick sections. It gels after a few seconds and can be uprighted and set on the shelf for drying.
L3685X white slip (left mug) has 5% more frit than Y (right). The frit is a melter, creating more glass bonds to adhere it to the body (it also hardens it and darkens the color a little). But the frit also increases firing shrinkage, 'stretching' the white layer on the body as the kiln cools (the slightly curled bi-clay bar demonstrates that). However the glaze, G2931G, is under some compression (to prevent crazing), it is therefore 'pushing back' on the white slip. This creates a state of equilibrium. The Y slip on the right is outside the equilibrium, it flakes off at the rim because the bond is not good enough. Adding more frit, the other side of the balance, would put the slip under excessive tension, reducing ware strength and increasing failure on exposure to thermal shock (the very curled bi-clay bar in the front, not this clay/slip demonstrates the tension a poorly fitted slip could impose).
Out Bound Links
In Bound Links
The story of how Ravenscrag Slip was discovered and developed might help you to recognize the potential in clays that you have access to.
Ask yourself the right questions to figure out the real cause of a glaze crawling issue. Deal with the problem, not the symptoms.
In ceramics, this term can refer to a number of things: -A clay slurry poured into molds to be cast into shapes. The slip is deflocculated to minimize water content and fine tune viscosity. The deflocculation process involves using special chemicals that enable you to create a fluid clay-water slur...
The practice of applying glazes to dried ware and firing in one operation. Obviously this is going to save money on energy. But it introduces extra problems also. In general, the thicker and heavier the ware and the greater its dry strength the greater the chance that it can be glazed easily in the ...
Mocha diffusion is a technique of layering slips onto ware so that the edges of the upper one bleed outward into the lower. An alkali/acid mechanism is employed. The lower layer is a typical water based slip (usually white or cream) that is gelled enough to stay stable on the ware and wet long enoug...
By Tony Hansen