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A white or colored slip applied to clay as a coating. The term "slip" is often used interchangeably with this, but we think of slip more as a decorative material/process. The tile industry uses the largest volumes of engobe by far, these are employed as opaque barriers between less-than-white bodies and their glazes. Engobes are so valuable to the tile industry because they enable the use of locally available (and therefore much less expensive) red and brown burning bodies (the white engobe provides a surface on which glazes can have the same brilliant colors as on porcelain). The use of these locals clays has another key advantage: red burning bodies vitrify at much lower temperatures, resulting in energy savings. There is another area of cost saving: glazes need less zircon opacifier (one of the most expensive raw materials used in ceramics).

Getting the consistency of the engobe right is difficult when applying to leather hard ware (the best time). If you are a potter, the preparation and use of an engobe may be the most delicate exercise you have ever done in ceramics. But the reward is worth it. And once you learn, it is easy. You need a good mixer. A variable speed kitchen blender can even work. The engobe must be gelled to the degree that it is thick enough to go on evenly, drain well and hang on in the right thickness. You can achieve the thixotropic gel needed by mixing the slurry thinner-than-needed (e.g. 1.4 specific gravity if it is a highly plastic engobe) and then gelling it using epsom salts or vinegar. Your mixer must be vigorous to mix the slurry well but not too much so that it pulls in air bubbles (important because they will not surface because the slurry is gelled). It is easy to over-gel the slurry, you can deal with this by holding back some ungelled material and adding it or by adding a tiny amount of deflocculant (like Darvan) to re-thin it. Your engobe will change consistency on storage (likely thinning) so you will need to readjust as needed. Be careful when pouring engobe into another bucket that has just been washed, any water in the bottom of that bucket will become a layer on top of the engobe and affect your ability to dip with a clean line. This process rewets ware so it can add a day to your drying procedure. However the benefits can also accelerate others parts of the process.

In traditional ceramics application is usually done at the leather hard stage (followed by various decoration techniques), whereas in tile it is applied to dry pressed ware as a curtain over a continuously moving production line (or as a powder layer during pressing). “Engobe” is a more specific term than “slip”. Potters use slips for decorative purposes, they are normally made from a clay of contrasting color (with added feldspar, bentonite, etc). Potters often traffic slip recipes, people just try a bunch until they find one that works with their technique or one that they can adapt to (they do this without thinking much about the recipes or mechanics of how they work). Industrial engobes, on the other hand, are usually designed for a specific body or body type, technicians creating and maintaining recipes are faced with a balancing act and constant production issues on multiple fronts. The engobe must have enough flux to fire-bond to and be thermal-expansion-compatible with the body below (and the glaze above) but not so much as to compromise the opacity. It also must match the firing shrinkage.

Bonding is a key consideration when using engobes. The thicker the layer the more issues there will be. The challenge starts at the application and drying stage. When applied to leather hard ware the engobe must bond mechanically to the surface during drying and shrink with the body. To do this it needs enough clays and binders to hang on but not so much that it shrinks excessively and cracks during drying.

Industrial engobes have recipes that look much like glazes, but with less frit. They must melt enough to glass-bond well to the body surface during firing. Since the thermal expansion of an engobe should be tuned to be slightly lower than the body, it must adhere well to stay on. With too much frit (or other fluxes) an engobe will begin to become translucent. Thus it is common to make a line blend of tests to find a compromise between the best bond and adequate opacity. Engobes have enough plasticity that drying shrinkage bars can be made.

Engobes need to be thermal expansion compatible with the body so that stresses coming with temperature change during service do not lead to flaking off in use. Of course, table ware will experience much more thermal stress than will floor tile, for example. When glaze is applied over an engobe, it is important that the thermal expansions complement each other. In tile, for example, the thermal expansion of the slip needs to between that of the body and the glaze.

Engobes must also match the firing shrinkage of the body. Obviously, if the body shrinks more or less than the engobe stresses are going to be introduced that can severely test the bond. This is a serious concern because engobes contain a flux and therefore, by definition usually have a higher fired shrinkage (7 or 8% is easily possible). The body, the other hand, might only be 5% (although vitreous bodies will be more). Test bars can be fired at a range of temperatures to measure the fired shrinkage of the engobe (or slip). Any changes made to the recipe to adjust for fired shrinkage need to be done in the context of preserving the thermal expansion, fired opacity and drying behavior.

If you do not have lab testing equipment it is best if the firing character of the engobe is as similar as possible to that of the body (vitreous engobes on vitreous bodies, non vitreous ones on non vitreous bodies). Such matching produces the best chance of compatibility in firing shrinkage and thermal expansion. It is possible to apply vitreous engobes to non-vitreous bodies if a way is found to reduce the firing shrinkage of the engobe (and reduce translucency using zircon). It is easy to imagine an engobe shrinking more than the body (and pulling on it) whilst being covered with a glaze that is under compression (thereby pushing on the engobe). The bond between body and slip will be seeking relief (which the first mechanical or thermal stress will be happy to provide!).

Potters have options that are prohibitive in industry (because of cost and difficulty). For example, high percentages of zircon opacifier are added to maintain opactity for thinner applications or for higher flux percentages. Some slips contain high percentages of bentonite (up to 10%) to make them very sticky and hard on drying. These obviously dry much slower but, surprisingly, with less cracking than you would expect. Other recipes employ exceptionally high gum contents in the water (as much as will dissolve in boiling water), but these dry slowly. This is another "crow-bar" approach to the challenge of getting the slip to dry-bond well with the body.


Too much frit in an engobe and it will lose opacity and whiteness

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.

A small laboratory variable speed propeller mixer

If you are at all serious about testing glazes and clay bodies, you need one of these. There are other methods, but nothing else comes close to this. These are expensive new, this one was more than $1000. But you can get them used on ebay.com. I adapted a mount (to give it vertical adjustment) from a hardware store. Propellers are also expensive, but you can design and 3D print them yourself or have them printed at a place like shapeways.com.

The same engobe. Same water content. What is the difference?

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.

Shivering on a transparent over an engobe

Example of a glaze (G1916J) shivering on the rim of a mug. But the situation is not as it might appear. This is a low quartz cone 03 vitreous red body having a lower-than-typical thermal expansion. The white slip (or engobe) has a moderate amount of quartz and is thus put under some compression by the body. But the compression is not enough to shiver off (e.g. at the rim) when by itself. However the covering glaze has an even lower expansion exerting added compression on the slip. This causes a failure at the slip-body interface.

Adding water actually made this white engobe run less? How?

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.

One small pinhole in a terra cotta mug and we have a problem

This is L3724E terra cotta stoneware. The inside slip is L3685S, a frit-fluxed engobe that is hard like the body and attaches well to it (engobes are often insufficiently fluxed). The glaze (G1916Q) is Frit 3195, Frit 3110 and 15% ball clay. The body has about 3% porosity, enough to make very strong pots. However that porosity is still enough to absorb water (and coffee). Although not too visible here, the pinhole in the inner surface has enabled absorption and there is a quarter-sized area of discoloration below the glaze. The piece could possibly be fired a cone higher, but testing would be required to see if the slip is still firing-shrinkage and thermal-expansion compatible with the body and that the body would not be over-fired. A better solution is adjust the firing curve to heal the glaze better. High temperature stoneware can easily have a 3% porosity also, so this is not just a low fire issue.

Double-slip layer incised decoration: A challenge in slip-body fitting

An example of a white engobe (L3685T) applied over a red clay body (L3724F), then a red engobe (also L3724F) applied over the white. The incised design reveals the white inter-layer. This is a tricky procedure, you have to make sure the two slips are well fitted to the body (and each other), having a compatible drying shrinkage, firing shrinkage, thermal expansion and quartz inversion behavior. This is much more complex that for glazes, they have no firing shrinkage and drying shrinkage only needs to be low enough for bisque application. Glazes also do not have quartz inversion issues.

Bleeding underglaze. Why?

This cobalt underglaze is bleeding into the transparent glaze that covers it. This is happening either because the underglaze is too highly fluxed, the over glaze has too high of a melt fluidity or the firing is being soaked too long. Engobes used under the glaze (underglazes) need to be formulated for the specific temperature and colorant they will host, cobalt is known for this problem so it needs to be hosted in a less vitreous engobe medium. When medium-colorant compounds melt too much they bleed, if too little they do not bond to the body well enough. Vigilance is needed to made sure the formulation is right.

How much does a glaze need to melt before it sticks to the body well?

The back flat side of balls of 1916J and Q low fire glaze that melted into a dome shape after being fired to 1550F. They have been turned over to see the back side (the front side is still stained by volatilizing carbon). Clearly they have reached zero porosity and are beginning to melt, yet they have not adhered to the vitreous porcelain tile! This demonstrates the degree to which an engobe must melt to secure itself well to the underlying body.

4 good reasons to consider making your own underglazes

Commercial underglaze colors fired at cone 8 in a flow tester (this is another good example of how valuable flow testers are). Underglazes need to melt enough to bond with the underlying body, but not so much that edges of designs bleed excessively into the overlying glaze. A regular glaze would melt enough to run well down the runway on this tester, but an underglaze should flow much less. The green one here is clearly not sufficiently developed. The black is too melted (and contains volatiles that are gasing). The pink is much further along than the blue. And cone 5, these samples all melt significantly less. Clearly, underglazes need to be targeted to melt at specific temperatures and each color needs specific formulation attention. Silk screening and inkjet printing are increasingly popular and these processes need ink that will fuse to the surface of the body.

The value of a white vitreous engobe over terra cotta at cone 03

At cone 03 many terra cottas will fire quite dense and stoneware-like. The lip of the mug on the left is covered with a vitreous white engobe (L3685U) under the glaze (G1916Q). Red bodies are much stronger at low temperatures, but do not lend themselves well to the bright glaze colors that work so well at that range. Putting an engobe on as a base enables decoration with colored slips and a clear over glaze.

How can you test if an engobe fits your clay body?

This is part of a project to fit a fritted vitreous engobe (slip) onto a terra cotta at cone 02 (it fires harder there). Left: On drying the red body curls the bi-clay strip toward itself, but on firing it goes the other way! Right: Test bars of the white slip and red body compare their drying and firing shrinkages. Center back: A mug with the white slip and a transparent overglaze. Notice the slip is going translucent under the glaze. Why? It is too vitreous. That explains how it can curl the bi-clay bars toward itself (it has a higher fired shrinkage). So rather than add zircon to opacify the slip, it is better to reduce its frit content (thereby reducing its firing shrinkage). Reducing the frit in the slip will also make it more opaque (because it will melt less). Front: A different, more vitreous red body (having a frit addition) fits the slip better (the strips dry and fire straight).

I want this engobe to gel in ten seconds. Why?

It is going to be applied to leather hard earthenware and it needs to be thixotropic. Why? I do not want it to run down from the rims of the mugs after dipping. The process: Stir the engobe, pour-fill the mug, pour it out and push it upside down into the engobe. If I can pull it back out before the 10 seconds is up I get a perfectly even gelled layer that does not move. A good test is to stir it then pull out the spatula slowly. If it hangs on in a even layer with only a few drips it is perfect. I also I tip the measuring cup to check it is gelling. Achieving this behaviour requires very careful additions of epsom salts (and thorough mixing between). As the slip approaches this 10 second threshold even a slight bit more salts will turn it into a bucket of jelly! I almost always overdo it! So I keep some thinner slip aside to dilute it down to perfect, not much is usually needed.

Inner engobe separating at the rim

This is the only place we have noted separation with this engobe and body. The engobe was gelled and the piece was drained upside down. So this edge was thicker. The separation occurs to the convex contour a couple of mm down (and breaks away to that point). The engobe is more plastic that the clay and thus shrinks more. Normally the extra stickiness from the extra plasticity is an asset, but not always.

Potters can learn from how glazes are fit on ceramic tile

These are thermal expansion curves for body, engobe and glaze (from a dilatometer, a device that measures it against increasing temperature). The upper line is the body. The center line is the engobe. The lower line is the glaze. The ceramic tile industry is very conscious, not only of glaze-fit but also engobe-fit. Engobes (slips) are employed to cover brown or red burning bodies so they glaze like a porcelain. Typically technicians tune the formulation of the engobe to have an expansion between the body and glaze. The body is highest so that during cooling, as it contracts, it puts a squeeze on the engobe (the engobe thus never finds itself under tension). The glaze has the lowest expansion, it is under a state of compression by the engobe (and slightly more by the body). This equilibrium enables the tile to wear for many years without crazing or shivering. Chart courtesy of Mohamed Abdelmagid.

How to test compatibility between engobe and terra cotta 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).

An engobe is shivering off the rim of a low temperature mug

Classic terra cotta bodies are not vitreous, so engobes used on them need to have similar low fired shrinkage. But when terra cottas are fired above cone 04 they start to mature and fired shrinkage increases quickly, flaking off engobes that do not have sufficient added frit (to increase their shrinkage). Even if an under-compression engobe can hang on, the extra stresses of an overglaze of lower thermal expansion can compromise the engobe:body bond. That has happened on the center mug. That engobe has less frit (10% vs. 15% for the others). The clear glaze on the left has high thermal expansion and is crazing, while the engobe:body bond can tolerate that it, is obviously not desirable.

Out Bound Links

  • (Glossary) Thixotropy

    Knowing about thixotropy will enable you to mix a ...

In Bound Links

By Tony Hansen

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