Creating a Non-Glaze Ceramic Slip or Engobe
Section: Glazes, Subsection: Slip, Engobe
Description
It is difficult to find a slip that is drying and fired compatible with your body. It is better to understand, formulate and tune your a slip to your own body, glaze and process.
Article
Non-glaze slips 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 glaze. Many potters are adapting this age old process to stoneware and porcelain. Industry, especially the tile industry, routinely applies slips and engobes (e.g. white engobes are put on darker burning dry tiles before applying glaze over top). Slip recipes don't travel well at all and are often not well understood. Local materials are typically involved, slips tend to be very body-specific and there are different factors to consider depending on whether slips are applied to leather hard or dry ware, what temperature they are fired to, what glaze and body they are paired with. The location-specific factors make it more practical, even necessary, to understand some simple principles and create or tune a slip to fit your purpose.
The major problem with slips is compatibility with the body, if the slip does not shrink at the correct rate or amount during drying and firing then cracks will develop. No matter what logic or theory might indicate or what others might advise, if cracking is occurring then you need to react by adjusting the formulation of the slip in the right direction (or possibly adjusting the way the slip is prepared or applied). Different factors are involved in attaining compatibility for firing and drying and a change that improves one may also detrimentally affect the other.
At first, the ideal solution might seem to be a slip made from the same recipe as the body (with or without colorants). However you may not be able to achieve good fired adherence if the body does not vitrify well and it will not likely be possible to avoid drying cracks (e.g. slips contain much more water than the leather hard clays onto which they are applied so they will shrink more, slips need to shrink but the dry body onto which they are being applied has already shrunk).
Adherence, Suspension, Hardness
- Non-glaze-slips 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, 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.
- 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 unstable surface). Bentonite/smectite/hectorite type materials are the best suspending agents, only 1-2% is sometimes needed and this has minimum impact on other properties (although drying time will slow and shrinkage will increase somewhat). Kaolins and ball clays will benefit best working properties and hardness (but they also increase drying shrinkage greatly). 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).
Fired Adherence
- The fired slips 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 the body to securely stick it on; one without can only hang on mechanically to any available surface irregularities. A vitreous body will also supply some glassy fluxes to adhere to a covering slip. If a slip is not glassy and the body is not vitreous then it is very important to make sure that the slip has the same fired shrinkage and thermal expansion as the body (even slight differences can compromise the bond). It is simple to test the integrity of and chip resistance of the bond by applying a thick slip layer to a test tile).
Dry Shrinkage
- Slips exhibit drying shrinkage just like clay bodies. The drying and dry bond between slip and body is fragile at best.
- If it is being applied during leather hard stage it is thus important that the slip shrink together with the body and the same rate and magnitude.
- 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. 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 (again, if clay content is too low the slip may not be compatible with the fired body, some non-plastic or coarser particled clays should be used).
- Compatibility can be tested by simple observation, it is easy to tell the difference between slips the shell off because they shrink less (when being applied to leather hard body) or crack because they shrink more.
Fired Shrinkage
- Slips have 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 curves need to be very similar. Compatibility is simple to test, just apply a thick layer of slip on a flat smooth surface of the body, fire it, then try to chip it off. If it does come off, determine if the bond is the problem first. If not then it needs to shrink more or less, adjust in the right direction and test again.
- The fired shrinkage of a slip can be increased by use 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.
Thermal Expansion
- To function together as a fired object body and slip must have a similar thermal expansion. This means that 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. The covering slip on a ceramic piece is a separate entity and has its own coefficient of thermal expansion independent from the underlying body. If sufficient incompatibility exists, sudden heating or cooling will overcome the slip-body bond and the slip can flake off.
- If a glaze is applied over a slip it is a more complex situation. The slip is acting as a buffer layer and it must be expansion compatible with both the underlying body and the overlying glaze. Normally it is best to adapt the glaze to the body first and test it well to assure compatibility. Work on adapting the intermediate slip layer can then proceed on the assumption that body and glaze are already matched. However note that glazes normally need to be under compression to achieve fit and an intermediate layer will have to be well adhered to the body to hang on despite compressive forces from the overlying glaze.
Other Factors
- Slips should exhibit a degree of maturity or vitrification in keeping with their intended surface character. While slip-glazes melt like normal glazes, nonglaze-slips normally have the flux content tuned to develop a surface character somewhere between body and glaze. As noted above, a slip that is more vitreous than the underlying clay body will adhere better.
- Slips develop characteristic fired surface qualities. They can be smooth, silky, glassy, textured, dry, stippled, etc. You control this by the degree of melt, slip makeup and application methods.
- A slip's flow characteristics, water content, stickiness, gelling properties, and drying speed enhance or degrade its application properties (and therefore ware quality).
As noted, the above factors can be divided between physical and fired considerations. If you need a slip for a particular body, the first step is to determine its purpose, then propose a simple recipe and begin testing. It is usually best to concentrate on matching physical properties first, then fired, then come back to fine tune physical again. Let's make a slip for dipping and painting on leather hard stoneware surfaces that may or may not be glazed over. We need good covering power to hide dark brown body color where necessary and the surface should have just a slight sheen so that it gives a pleasant feel to the touch.
A Slip Recipe for Cone 10
The most fired- compatible slip would obviously be the body itself made into a slip, however this is not the case for drying compatibility. We can view a basic slip as a mix of clays with enough feldspar to impart the necessary maturity and the rest filled out by silica. The real trick is to choose the right mix of clays (e.g. ball clay, kaolin, bentonite) to provide a drying-shrinkage compatible material. Here is what I would start with:
30-35% feldspar or Nepheline Syenite
This is more feldspar than a porcelain body would have and will thus provide adequate melting to produce a surface having a slight sheen. For a cone 6 slip, this would need to be about 50% or more if no boron frit is used.
1-2% 325 mesh Bentonite
This will help harden the dry surface to make it resistant to smudging during handling. Bentonite has an incredibly small particle size and it is very effective to increase drying shrinkage, suspend the slurry, and harden the dry surface. This small amount will not impact fired properties. This ingredient can be reduced or increased to allow easy day-to-day fine tuning to compensate for tendencies to flake or shrink off.
20-25% Ball Clay
This will have the biggest single effect on the slip's physical properties. It will suspend and harden it and give it a sticky nature. It will, however, move the color toward off-white since it contains some iron impurities.
20-25% Kaolin
Its larger particle size will complement the ball clay well and impart a creamy texture to the slurry. The two materials together can be thought of as the 'clay complement'. Their amounts can be traded off one against the other to tune the slips physical properties during development. Plastic, non-plastic, and calcined kaolins are available to give you considerable control over the slips hardness, suspension, and shrinkage properties without impacting fired results.
20% Silica
This is a filler and helps to give the slip thermal expansion properties more compatible with typical bodies that contain lots of silica. If glazes tend to craze over the slip or the slip weakens the body, silica is important.
5% Zircopax
This will help give the slip extra whiteness and opacity so that it covers well. Its fine particle size will enhance the slip's creaminess and working properties.
.5% Gum
This will impart extra dry hardness or make it paint better (but it will slow down drying).
To challenge the slip-body bond, apply a thick layer of the slip to the side of some test pieces that are at the correct stage. Allow to dry. If the slip shrinks more than the body and forms little islands with curled up edges, then it's too plastic (take out some bentonite or trade off some plastic clay for less plastic). If the slip has not shrunk, attempt to scrape it off with a knife. If it falls away easily, it is not shrinking as much as the body (add bentonite or trade off a less plastic clay for a more plastic one).
Next, fire the ware and break it into small pieces. Examine shards closely to see if the slip is flaking off. If it is, add more feldspar to develop a better slip-body interface or try to match the body's firing shrinkage patterns better by adjusting the kaolin-ball clay mix up and down.
A Few General Notes About Using Slip
- Slips are more opaque than glazes and have great 'hiding power' (even in a very thin layer) that enables them to cover the body and mask its color and surface character. This hiding includes 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 a slip will not prevent the deposit of soluble salts (from the body) on its surface as water evaporates and leaves them behind.
- While slips can act as mediums for stains and metal oxide colors most stains do not develop their color well in slips because of the lack of a glassy phase. Color is often a matter of chemistry and interaction between glaze and colorant but since slips do not melt as much so the chemistry does not happen as spontaneously. Since body compatibility concerns are so complex with slips there is often not enough flexibility to adjust chemistry for stain compatibility. A simple solution is sometimes available: use a body stain. Chemically incompatible slip-color combinations can be crow-barred into working by saturating the slip with color, but this can be expensive (and makes little sense when a slip recipe change would be more effective). Another aspect to consider is that since slips are opaque the only stain particles that influence color are those exposed at the surface (this is a waste of money).
- If you have the facilities to slurry and dewater a small amount of the slip and make dry and fired shrinkage test bars, this is an excellent idea. This way you can compare the figures directly. I have found that slips with 1-2% less drying shrinkage than the body function well.
- As noted, it is difficult enough to get glazes to fit properly on a clay, but putting a slip of unknown thermal and fired shrinkage properties between a glaze and clay could open a real 'Pandora's box' for you. If you are determined to make it succeed, test the glaze fit on the body first (by hot water/ice water immersion cycling to reveal crazing or shivering or by comparing the fired strength of glazed bars with that of unglazed bars). Adjust the glaze if necessary and repeat. Next, match the slip to the body and do adhesion and shattering tests to be sure there is a good fired bond. Finally, test the combination as you did the glaze alone.
- The body surface needs some roughness to give the slip a place to grab on to. Additionally, contact between the body and the liquid phase of the slip is important to establish a good initial bond (take this into account when spraying). If you would like a very creamy texture, consider ball milling the slip thoroughly.
- Slip formulation is normally done on the recipe level. Ceramic calculations are not really applicable because non-glaze slips don't melt.
- However, be careful about putting metal oxides in slips that will be exposed to food or drink. The metals will leach out more readily than with a glaze since oxide particles are not dissolved into a glassy matrix.
Creating your own slip is not really all that difficult, but it will take time, determination, and a methodical approach with plenty of testing. Like glazes, it is far better to one slip you understand and control than mess around with 10 that you don't understand and don't work?
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Authors
- Tony Hansen (Owner)
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