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Creating a Non-Glaze Ceramic Slip or Engobe

Section: Glazes, Subsection: Slip, Engobe

Description

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.

Article

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.

Because slips usually need to be paired with the specific stage of the process and body they are used on, their 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 and each of these has multiple aspects (if local materials are employed in the engobe there are more issues). The major challenge with slips (or engobes) is 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 engobe. Obviously good adherence to the fired body is paramount, so the engobe is going to need 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 engobe that can hang on without a good interface).

Dry Adherence, Shrinkage, Hardness; Slurry Suspension

• Non-glaze-slips (engobes) have physical working properties that determine how well they paint, dip or spray; how well they adhere, how they drain after dipping, how fast they dry to reach the same water content as an underlying leather hard body (if applied at this stage), how thick they can be applied with each coat, whether double-coats can be applied, how well they stay in suspension during storage, how much they tend to gel, etc.
• It would seem logical that the body surface needs some roughness to give the slip a place to grab on to. While this is true with glaze application over bisque ware, it is not true with slip over dry body. The slip is wet and any existing  on the dry body surface is going to be smoothed anyway by the action of the wet slip. Slip and body are both clays and they are sticky and thus have a natural affinity.
• Normally the clay ingredients act to suspend non-plastic particles like frit, feldspar and silica, they impart adherence properties to the leather hard or dry clay surface and they harden the dry surface enough to withstand handling (a slip that dries hard resists smudging and smearing when pieces are handled; a soft one has a powdery surface). Kaolins and ball clays are most common and will produce the best working properties and hardness (but they also increase drying shrinkage). Bentonite/smectite/hectorite type materials are the best suspending agents (if of high quality), only 1-2% is sometimes needed and this has minimum impact on other properties (although drying time will slow and shrinkage will increase somewhat). Organic gums and binders can also be used to improve hardening and adherence (they will also slow drying and may introduce glaze pinholing problems if too much is used, as they generate gases of decomposition during firing).
• Both glazes and engobes shrink during drying because they contain clay. The drying and dry bond between slip and body is fragile at best. Engobes are less glaze-like and more clay-like, thus is would seem that there will be more problems with cracking during drying (due to the shrinkage). However, in practice you must adapt the recipe of a slip to shrink the amount you want (whether you are applying it to wet, leather hard, green ware). This will involve using a calcined or less plastic clay if necessary. Obviously, the same slip is not going to adhere well in all situations, you have to determine at what stage to apply it and tune it to that.
• If it is being applied during leather hard stage the slip must dry to the same water content as the body and then shrink together with the body, obviously that final shrinking period is going to reveal any difference between the two (well actually not quite, if you intend to apply the glaze over the dry slip layer that will rewet the slip and test its bond with the body again).  Drying compatibility can be tested by simple observation, it is easy to tell the difference between slips that shell off because they shrink less (when being applied to leather hard body) or crack because they shrink more.
• Conversely, if slip is being applied to dry ware it needs to shrink as little as possible. To achieve this it will be necessary to either minimize clay content (especially fine grained clays), use low-shrinkage clay or clay-like minerals, or employ a portion of calcined clays, apply the slip with the lowest possible water content or apply the slip to heated ware (the speed initial dewatering). Kaolins shrink the least, ball clays much more. Kaolins are available in many ultimate particle sizes, the larger the size the lower the shrinkage will be (and the less hard the dry glaze will be). Reducing drying shrinkage by the judicious choice of clay (vs. elimination of clay) is  more likely to maintain fired compatibility. Organic binders can be employed to impart or augment hardening and adherence properties, but their burn off can bring trouble during firing (with slip adherence and glaze imperfections).
• A slip will not prevent the deposit of soluble salts (from the body), they will simply migrate through the slip and be left on its surface as water evaporates and leaves them behind. You thus need adequate barium carbonate in the body to precipitate them.

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).

• The fired slip layer may be either loosely or tightly bonded to the underlying body. A slip containing glassy fluxes that melt and flow will form an interface with a vitrified body to securely hold it on (the body can also supply some glassy fluxes to form a better interface to the covering slip). If the body is not vitreous then even a glassy bonded engobe can disengage simply by taking some of the body surface with it (the forces involved with a thermal expansion difference between between body and slip are certainly up to this task). If the slip is not glassy then it can only hang on mechanically to any available surface irregularities. Thus it is very important to make sure that the slip has the same fired shrinkage as the body (even slight differences can compromise the bond).
• An engobe that is too vitreous can crack and pull away from the body in some places. How? It is normal for high-clay glazes to form some cracks as the temperature rises in the kiln, this is a by-product of the shrinkage associated with the early stage melting and densification of the ceramic powder. However as the glaze melts the cracks are healed back together. But with a slip the liquid healing phase never arrives. There is thus a tightrope to walk between an engobe that is vitreous enough to form a bond but not so vitreous that it suffers early-stage melt shrinkage.
• There is a difference between the integrity of the bond when a tile comes out of the kiln and over the long haul of it's useful life. If the engobe and body did not shrink together there will be stress, if the kiln has not revealed it a simple chipping test might. The ability of a thickly applied slip to hang on through a firing and also resist mechanical chip testing is an even better test of firing shrinkage compatibility.
• A slip has a characteristic firing shrinkage just as a body does. Body and slip must shrink together during cooling in the kiln in a way that does not introduce tension that can weaken the slip-body bond. Not only does the total shrinkage need to match, but the shrinkage curves need to be similar. Stress the bond to test and determine if the bond is the problem first. If not then it is a shrinkage mismatch, adjust in the right direction and test again.
• The fired shrinkage of a slip can be increased by using more fluxes to impart greater vitrification or melting, or by using a finer particle sized clay. In many cases it is practical to make a pugged sample of the slip and make test bars for measuring the fired shrinkage, this is an excellent way to evaluate quantitatively the drying and fired effects of recipe changes.

Thermal Expansion

• To function together as a fired object body and slip must have a similar thermal expansion. The covering slip on a ceramic piece is a separate entity and has its own coefficient of thermal expansion independent from the underlying body. Thus when the object is heated or cooled quickly during use the minute amount of expansion and contraction that occurs must be similar in slip and body. If sufficient incompatibility exists, sudden heating or cooling will overcome the slip-body bond and the slip can flake off at vulnerable sites (like edges and rims).
• When a glaze is applied over a slip (which is practically always) more complexities arise. The slip is acting as a buffer layer and it must be expansion compatible with both the underlying body and the overlying glaze. How? Thermal expansion 'compatability' for a slip is different than for a glaze, a slip cannot be put under that same amount of compression because it does not have the same bond. So, of course, the overlying glaze cannot impose too much compression on the slip, it would need to be of a lower expansion than if it were applied to the body directly (or the slip-body bond can be compromised by compressive forces from the overlying glaze). Thus, the difficulty of matching a slip may be so onerous that making functional ware (with an engobe) that can survive constant heating and cooling maybe be impossible for you (a floor tile experiences only moderate temperature changes during it's service).
• It is simple to test the likelihood of the ongoing integrity of the thermal expansion tested slip-body bond: Douse a hot slip-covered (or slip and glaze covered) tile in cold water.

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. Thus drips, variations in thickness, roughness, pinholes, 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. 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.

• Consider ball milling your slip thoroughly.
• Slips are more opaque than glazes and theoretically have better 'hiding power' to enable them to cover the body and mask its color and surface character (including smoothing an otherwise rough or coarse surface, obscuring an undesired color, hiding specks and surface imperfections (like those caused by iron impurities), tightening and sealing an otherwise porous and soft surface, and providing a whiter surface to brighten glaze colors). However in actual practice slips applied too thinly can have surprisingly little covering power.
• Spraying can work well but you need to have a sprayer that can function with a slip of the lowest possible water content. Gravity fed devices are good (the can is above the nozzle). If the slip is too viscous however it will lay down as droplets and give an uneven surface. Greater air pressure can help, but that will create more overspray and breathable mist and produce a pebbled surface. It can be a challenge to get a thick enough laydown that covers body imperfections. On one hand the multiple coats needed consume extra time while attempting to spray a thick-all-at-once layer can produce drips and a water-logged fragile item, not to mention a slip that might crack and pull away on drying. You may find that heating the ware (assuming you are spraying dry (unbisqued ware) will help dry the slip and enable you to apply a thicker more even and smoother layer. Like glaze, it is best to have a process where the slip dries quickly.
• Pouring the slip can work very well for some shapes (e.g. continuous waterfall type application). But it may only be practical for one coat application: your slip consistency, ware temperature, manner of application all must be right to avoid drips and overlap layers.
• Dipping is not entirely practical for engobe application. While dipping a bisque fired object using tongs is a simple procedure that works very well, for a dry item it is not practical because items do not have the strength to be held by tongs and even if they did the water from the slip will cause the object to crumble from the stress of being handled. In the past heavy stoneware was done this way, it had the strength to survive the handling. However these days no one can afford to fire heavy objects and quality demands are higher.
• Engobes can be brushed on in multiple layers at various stages of the process, but this is labour intensive and produces brushstrokes. The brushstrokes can be tolerated for visual reasons at times or a wide very soft brush can be employed to produce smoother surfaces.

The Slip Recipe

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 causes densification associated with melting (with it's accompanying surface disruptions). 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, this will give you more flexibility to adjust the recipe.

For high temperature you do not need to use frit, increasing the feldspar would be better. 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) and reduced it to 40 and increased the feldspar by the same amount). If it can be more mature, try 40 feldspar also (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. To this try adding 10-20% of a boron frit (try different frits, some are much better for bodies).

For more whiteness consider using a whiter kaolin or adding some zircopax (perhaps 5%, it will also impart more opacity). Use calcined kaolin for part of the clay complement if needed to cut drying shrinkage. 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, remember gummed slips dry slower and that is the last thing you need).

A Few General Notes About Using Slip

• An additional test for the slip body bond is simply to break a fired item into small pieces and examine the shards closely to see if fracture lines tend to prevail at the slip-body interface.
• While slips can act as mediums for stains and metal oxide colors, stains do not develop their color as well in slips (because of the lack of a glassy phase and because slips are opaque the only stain particles that influence color are those exposed at the surface).

Creating your own slip will take time, determination, and a methodical approach with plenty of testing. 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?

Out Bound Links

• (Materials) Ravenscrag Slip - Silty fusible low iron clay

In Bound Links

• (Articles)

  Ravenscrag Slip is Born

  The story of how Ravenscrag Slip was discovered and developed might help you to recognize the potent...

• (Troubles) Glaze Crawling
  Asking yourself the right questions to figure out ...

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