Engobes are high-clay slurries that are applied to leather hard or dry ceramics and fire opaque. They are used for functional or decorative purposes.
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, paint-on 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 the overlying glaze, they are applied by automated pour/dipping methods. 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 local clays has another key advantage: red burning bodies vitrify at much lower temperatures, resulting in energy savings. Another area of cost saving is that glazes need less zircon opacifier (one of the most expensive raw materials used in ceramics). However, there is a downside: This process rewets ware so it can add hours or a day to your drying procedure. However the benefits can also accelerate other parts of the process.
In terracotta and stoneware processes, engobes are most often applied to leather hard ware. Perfecting the rheology (specific gravity, viscosity and thixotropy) of a slurry is difficult, delicate. A good wall-mounted variable-speed propeller mixer is invaluable because you can set it to mix the slurry well but not so much that it pulls in air bubbles. The engobe needs a lower specific gravity to dry as fast as possible, be fluid enough to pour and drain well but have a high enough thixotropy to gel and stay in place without drips after application. It is best to have a recipe that recommends a specific gravity and the proportion of powder mix, water and Darvan (or other appropriate deflocculant) to produce that specific gravity (1.5 is a good for most potters). If the recipe is right, then after thorough mixing it should be fairly fluid (in need of a flocculant to gel it somewhat). If it is not fluid, measure the specific gravity (by the weighing method, not a hydrometer, they are useless for this). If it is high, add water, measure again and repeat till right. If low, adjust the printed recipe for next time (reducing the water proportion) and proceed (a lower SG will still work, it just dries slower). It is now a balancing act: Enough deflocculant that the slurry is fluid enough to apply evenly and drain well, enough flocculant that it gels after a few moments (so you do not have to wait while it drips-drips-drips). In a gallon-size batch this can mean additions of only drop(s) of flocculant or deflocculant to move it as needed. Trial application on pieces is the best way to determine if the rheology is right. The whole process really is magic once you learn it. One warning: If you overdo a flocculant addition when adjusting the slurry can immediately become lumpy. Do not panic, the addition of a little deflocculant and propeller mixing will bring it back.
Engobe consistency will change on storage so mix thoroughly and readjust the degree of gel on each use. Once it has stabilized to the ideal consistency the slurry will gel on storage and resist thinning without propeller mixing. It will also power mix to what appears to be too thin of a consistency, but will gel up quickly to working behaviour.
In traditional ceramics application is usually done at the leather hard stage (followed by various decoration techniques), whereas in tile it is applied to dust-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. By that term we mean that people often just try a bunch of recipes until they find one that appears to work (without considering any technical considerations). 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.
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. Strangely, engobes having a plasticity (and stickiness) considerably higher than the body seem to bond best during drying. For this reason it is common to see very high ball clay percentages and additions of as much as 10% bentonite (which slows down drying considerably).
Engobes need to be thermal expansion compatible with the body also. This is so that stresses coming with temperature change during service do not lead to flaking off in use. Of course, tableware 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 engobe needs to be between that of the body and the glaze.
Engobe recipes found online often look like glazes, but they should not. Engobes are like bodies, there should be little or no frit or boron source in the recipe (except at low temperatures). Excessive flux will compromise the most important attribute: opacity. More important, excessive flux will give the engobe a greater fired shrinkage than the body, introducing stresses that can severely test their bond and weaken the fired marriage. A vitreous engobe might have a fired shrinkage of 15%. Many stoneware bodies can be a little as 5-8%, vitreous bodies up to 10%. Earthenwares can exhibit a wide range of shrinkages depending on the firing temperature. Test bars of body and engobe can be fired at a range of temperatures to measure the fired shrinkages. Or, bi-clay strips can be made, these sandwich the body and engobe in a thin bar that curls during drying and firing to indicate fit or misfit. The lesson: Vitreous engobes on vitreous bodies, non-vitreous ones on non-vitreous bodies.
Engobes are opaque so that means a relatively thin layer has the potential to be as effective as a thick one. Thus it is advantageous to tune the rheology of the slurry to effect a thinner application layer. A big benefit is that the leather-hard ware will not soften so much and will stiffen much quicker for further finishing.
Potters have options that are prohibitive in industry (because of cost and difficulty). For example, high percentages of zircon opacifier can be added for whiteness and opacity. Highly plastic and stickier mixes, lower specific gravities or high gum additions can be tolerated (they dry much slower).
This cone 6 mug is made from a black clay (containing 10% burnt umber). The engobe on the inside is covered by a clear glaze. The color is the same as if the engobe were on a white or buff firing stoneware. Engobes get their covering power from the fact that they do not melt. If you see an engobe with lots of frit it will likely melt too much, be suspicious.
The shrinkage (and stickiness that goes with it) are important to adherence during application at the leather hard stage. And also to shrinking with the body during the drying process. But on bisque? No way.
This flake shivered off the rim of a low fire terra cotta mug. It is Fishsauce slip. It is about 2 inches long and has razor sharp edges. This is not the sort of thing you would want to be falling into your coffee or food and then eating! This flake did give evidence that it was loosening so there was little danger of me consuming it, but smaller flakes can go unnoticed. Slips (or engobes) must be drying compatible, have the same firing shrinkage, the same thermal expansion and be quartz inversion compatible with the body. It is easy to ignore all that and pretend that it works, but the bond between engobe and body is fragile at low fire and easily compromised by the above incompatibilities. Slips must be fitted to the specific body, glaze and temperature; that involves a testing program and often a little chemistry. I have documented online how to I adapted this slip to Plainsman Terrastone 2 using my account at insight-live.com.
Left is Plainsman M340. Right is M390. Each mug has been white engobed inside and half-way down the outside. The insides have been glazed using G2926B clear. The inside surface has more depth and has a richer appearance than you could achieve using a white glaze (especially over the dark burning body). The outside of the left one is Alberta Slip base GA6A using Frit 3195 (it produces a more stable glass of lower thermal expansion). The outside glaze on the right is the clear plus 4% iron oxide. This technique of using the engobe enables porcelain-like functional surfaces on the insides and striking visual contrast and character on the outside of the dark body mug.
This is how bad the fit can actually be. In the front is a bi-clay EBCT test strip of a grogged cone 10R sculpture clay sandwiched with a porcelain. After drying this bar was relatively straight. But during firing the porcelain has a much higher fired shrinkage and it pulls the bar toward itself. During cooling, the sculpture clay has a higher thermal expansion and it pushes from its side bending the bar further. This bar is a bomb of internal stresses, just waiting for a mechanical or thermal stress to bust it into a hundred pieces. Admittedly, putting a thin layer of this onto a piece of heavy ware is not going to bend it. But will it flake off when exposed to stresses (like freeze thaw, being put in an oven, having a hot liquid poured into it, being bumped).
The foot ring on these hard mugs has already been trimmed. At the stiff-leather-hard stage an engobe was applied to the inside. This rewet the bodies of the mugs, almost to the same point as freshly-thrown. But the handles did not get rewetted. To re-dry these mugs to the point of being able to turn them over will take 4-6 more hours. But by that time the handles will be bone dry. To prevent that I waxed them after trimming. That slows their drying down enough to keep them even with the body of the mug. To dry ware successfully the key is to keep all parts of a piece of the same water content throughout the process.
This is a white engobe (L3954B) drying on two dark burning cone 6 stoneware leather-hard mugs (Plainsman M390). Those lumps are on the left cannot be screened out, they are agglomerates. That slip has excessive flocculant (powdered Epsom salts are added to gel it so that it stays put on the piece after dipping). About 4 drops of Darvan were added to one gallon of the slurry, this immediately made it smooth and a perfect consistency for application. It remains stable on ware (without runs). Engobes require tight control to have the right viscosity and thixotropy (which can be achieved over a range of specific gravities (about 1.45-1.6). When they are right they are a joy to use, when they are not ware is ruined.
The handle on this mug joined the wall at an acute angle. The wet engobe was applied to the leather-hard piece and took hours to dry (rewetting and softening the whole piece in the process). During that time the engobe was able to exploit the join and penetrate into it (the crack diverted straight into the soft weakened wall). Notice how the engobe has pooled at the join in the other two mugs also (although they did not fail). Engobes typically have a high water content and experiences like this teach us to take precautions when using them.
The amber glaze on the outside of the left mug contains 20% super-low thermal expansion Ferro Frit 3249 as the melter. With no underlying engobe it can form enough of a bond with the body that it does not flake off at the rim (even though it is under excessive compression because its low thermal expansion). This flaking is called "shivering". The engobe, which does not melt like a glaze, has a more fragile bond with the body (and the glaze is pushing enough to make that bond fail). The mug on the right employs 20% Frit 3195 melter instead, producing a glaze that fits better. I hammered both of these rims repeatedly with a metal object to stress them, that one on the right definitely fits better.
These bi-body strips are made by rolling two clays together in a thin sandwich. Three porcelains are being compared to a very plastic grogged sculpture body. After drying (top) they curl a little, two toward the sculpture body and one, the most plastic of the porcelains, toward the white. But on firing to cone 8 they curl dramatically toward the porcelain side (because it shrinks alot more). Now imagine one of these porcelains is being used as a engobe on this body.
This black one is on a cone 6 buff stoneware. Because a thin layer works well with this high-opacity engobe it is possible for the slurry to be more fluid, less gelled. An immediate benefit of this is that it dries much more quickly (enabling handling of the piece within a few hours). Another benefit is a much more even coverage than would be possible with more a viscous consistency. The thinner layer also and has much fewer issues with flaking during drying. However application of the engobe take patience, perhaps a minute waiting for drips to stop and gelling to set in to hold it in place.
L3685U slurry was applied to the insides of both of these mugs. But on the left it is a "slip", on the right an "engobe". Why? The left mug only has a thin layer, applied by painting a gummed version on (at leather hard stage). On the right a gelled slurry was poured into the leather hard piece, poured out and the rim dipped (creating a much thicker layer with more power to impose its own drying and firing shrinkage). So it is much more important that the latter be compatible with the underlying body. The EBCT test is used to measure how compatible the body and engobe are.
The EBCT test bars (engobe compatibility) in the foreground demonstrate the issue (they sandwich the engobe and body as a bi-clay strip). After firing at cone 10R they have curled toward the engobe side, indicating that it is shrinking more. On the mug the engobe has done likewise, shrinking more than the body and creating a crack pattern.
This stoneware jar is made from a large particle size body with grog added. It was engobed on the inside and over the lip at the leather hard stage and bisque fired. Now it has been dip-glazed inside and out. Because the clay is so porous, lots of air must escape from inside the wall as the matrix soaks up water from the freshly applied glaze. But the air is being channeled into pathways and concentrated into surface irregularities created by the coarse particles in the matrix. At each of the escape sites a bubble appeared (then healed into a depression when it burst). But the engobe on the inside creates a homogeneous surface that distributes air escape uniformly, it does not have a single blister.
This engobe has high plasticity and is sticky, it is intended for use on leather-hard ware. The leather hard clay resists absorbing the water from the engobe (although it does soften somewhat) and just waits for it to catch up. Then the two shrink and dry together, forming a good bond. If the piece is dry, however, the wet engobe quickly slakes and wets the surface, pulling pieces away as it shrinks.
This is the L3954B engobe recipe but it has 15% Mason 6600 black body stain (instead of the normal 10% Zircopax for white). There is no cover glaze, yet it is durable and absolutely coal black (so a lesser stain % is possible). We have updated the mixing instructions at PlainsmanClays.com and Digitalfire.com pages (showing exact amounts for water, powder, Darvan) and the text on the glossary pages about thixotropy and engobes (read these again and learn to use the engobe process even better). This engobe base is designed to work on regular M340/M390 stonewares (not porcelains). This is exciting because these bodies are so much more robust in drying and much less expensive than porcelains.
Stains are fired, inert particles of a relatively large ultimate size. Unlike that, raw oxide powders, like iron or manganese, have much finer sizes and are thus extremely dirty to use. This plaster slab is being used to dewater these 15% black engobes for shrinkage testing. The slurry on the right has just been poured, the one on the left has just been peeled up (it was spread across almost the entire surface). Notice it has left no stain (the marks on the outer edge wash off easily). What does this mean? It means that using this engobe is much, much cleaner than using a body or slip colored using raw or burnt umber, iron, manganese or cobalt.
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.
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.
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.
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.
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.
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.
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 very viscous with a 1.48 specific gravity. Why? Because it was not thixotropic. The fix? I watered it down to 1.46 (making it runny) and added pinches of powdered epsom salts (while mixing vigorously) until it thickened enough to stop motion in about 1-2 seconds on mixer shut-off. But that stop-motion is followed by a bounce-back. That is the thixotropy. It is easy overdo the epsom salts (gelling it too much), I add a drop or two of Darvan to rethin it if needed. When the engobe is right it gels after about 10 seconds of sitting, so I can stir it, dip and extract the mug, shake to drain it and then it gels and holds in place. Keep in mind, this is a pottery project. In industry they deflocculate engobes to reduce water content. But a deflocculated slurry can still be gelled (if it is runny).
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.
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 (powdered), 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.
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.
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. One caution: The engobe needs to have the same firing shrinkage as the body, to use a vitreous engobe the body needs a frit addition to be vitreous also. If you put a vitreous engobe on a porous earthenware its firing shrinkage will put it under tension and the poor bond with the body will mean quick or eventual failure.
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).
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.
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.
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. It is the most valuable and frequently used tool in any ceramic bodies and glazes testing lab or classroom. These are expensive new, this Lightnin 1/20 hp variable speed cost more than $1000 many years ago, now it could be $4000! 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.
It is going to be applied to leather hard earthenware and it needs to be thixotropic (gelled when not in motion, liquid when in motion). 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 second gel-time is up I get a perfectly even 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. Achieving this behaviour requires very careful additions of powdered 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 (if that happens I add a drop or two of Darvan). This process works across a range of specific gravities (about 1.45-1.6), the higher the SG the trickier it is (but the faster it dries).
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).
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.
Terra Sigilatta is a sedimentation process used by potters to remove coarse particles from a clay powder. It enables burnishing and gloss surfaces without the use of glaze.
In ceramics, the zone of adherence between glaze to the underlying body is called the clay-glaze interface. The integrity of this interface is important to strength and functionality.
A ceramic compound meant to be applied to green or bisque ware and covered using a transparent overglaze. There are good reasons to make your own underglazes if you are in production.
A technique used by potters to decorate ware. Neutral and colored slips, clay in thick paste form, are trailed onto the ware (in raised lines and strokes) to create tactile designs.
During drying, clay particles draw together and shrinkage occurs. During firing the matrix densifies and shrinkage continues. More vitreous bodies shrink more.
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Cone 10 reduction black engobe for stonewares (not porcelains).
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