All Glossary

Slip Casting

A method of forming ceramics. A deflocculated (low water content) slurry is poured into absorbent plaster molds. As it sits in the mold, usually 10+ minutes, a layer builds against the mold walls. When thick enough the mold is drained.

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Details

Slip casting is the forming of ceramics by pouring or pumping deflocculated (water-reduced) clay slurry into plaster molds. In the process, the absorbent plaster pulls water from the slurry, and over a period (e.g. 20 minutes) a layer builds up against the mold surface. The slurry is then poured out and, as the item stiffens it shrinks slightly and pulls itself away from the mold enabling removal. This casting process is flexible, capable of producing fine delicate porcelain items yet heavy sanitaryware (they can cast pieces up to 50kg). Slip casting is typically the only option where complex-shaped pieces are needed.

The casting process has some compelling advantages over plastic or dust-forming methods. Consider:

• Many shapes are very difficult or impossible to produce any other way.
• Slip-cast ware can dry-shrink as little as 1.5% (compared to 6%-8% for plastic stoneware bodies).
• Porcelain can be made whiter and more translucent because the dirtier plastic materials are not needed.
• All clay can be reclaimed and put into slip for reuse.
• Slip is easy to magnetically purify and sieve, and its consistency can be adjusted easily for optimum workability.
• Slip-cast ware can be made much lighter and thinner walled.
• With some bodies handles and add-ons can even be applied to dry ware using slip and yet no cracking occurs!
• With control of your clay body recipe it is feasible to adjust it to get higher or lower thermal expansion, greater translucency, or to make a custom body for refractory or low-fire applications.
• The workflow consists of a series of small tasks often separated by period of time. This is such that a potter can integrate slip casting into his existing production to increase output.
• Slip casting can often be done by non-skilled people.

The sanitary ware industry produces the largest tonnage of products using this process. Water closets and sinks are made by casting porcelain and whiteware in complex many-part molds. Traditional plaster molds are heavy, cast sections are thick, and take considerable time to release and extract from the molds (e.g. 8-10 hours). However modern methods employ resin molds, automated mold-handling and filling equipment, pressurization of the slip feed, heated slurry, air-assisted drying, vacuum dewatering of molds, etc to reduce casting cycle time to as little as 20 minutes!

Many types of powdered ceramic can be made into a slurry and cast (except for clays that are high in soluble sulphates or gel on attempts to deflocculate). If the slurry can be suspended and deflocculated, it shrinks enough on drying and has enough strength to hold itself together as it pulls away from the mold, it can be usually cast. Even non-plastics like calcined alumina and silicon carbide can be cast by incorporating small additions of plasticizers and binders. Since the whitest burning clays have low plasticity, casting lends itself to producing white-burning ware.

In industry, casting rate is important. Items need to cast and be extracted from molds quickly (shrink away and hold together). Molds need to be dewatered rapidly. Aside from the above-mentioned automation, much can be done to fine-tune the balance of permeability and plastic strength of a body. It is important to understand the role of each material in the body recipe. Compared to a plastic body a typical industrial casting body has more kaolin, less ball clay, and usually no bentonite. Ball clays and kaolins of the largest ultimate particle size are preferred (since these are more permeable to water passage). The above being said, bodies can actually cast and release too fast, causing issues in production.

Casting bodies that are relatively non-plastic can have very low drying shrinkages compared to plastic ones, ware can be dried quickly with little likelihood of cracking. Separately cast, leather-hard sections can even be glued together with slip and the pieces can be dried without issue. However, as noted, non-plastic ware is often difficult to release from molds - especially if walls are thin or shapes are complex. And it crumbles when cut, even with sharp tools. Some potters thus use a dual-purpose body recipe (e.g. the casting version having 1% bentonite and the plastic version having 3-4% bentonite). The extra leather hard strength and trimability are considered well worthwhile despite much longer casting times.

The casting process does not lend itself to grogged bodies. The grog will not normally suspend well in the slurry. And it adversely affects the rheology of the slip. And it would also be abrasive on mixing and slip-handling equipment.

A common issue with production casting is the recycling of scrap. Typically batch recipes call for a percentage in keeping with how much is generated by the production process. Scrap is commonly less plastic than a virgin mix. The exact reason is less researched than just responding to it as appropriate. One explanation is that the super fine plastic particles get preferentially pulled out of the slurry during time in the molds. The addition of a small amount of bentonite, e.g. 0.5-1%, is the simplest solution.

3D printing technology has many applications to casting, especially for mold-making and tooling (see linked projects below for examples).

Related Information

Delflocculation accomplishes the seemingly impossible

It is possible to get that 3000g of porcelain powder into this 1100g of water! And the fluid slurry produced, 2250cc, will fit into the front container. How is this even possible? The water has 11 grams of Darvan 7 deflocculant in it, it causes the clay particles to electrolytically repel each other! An awareness of “this magic” can help give you the determination to master deflocculation, the key enabler of the slip casting process. Determination? Yes, the process is fragile, you must develop the ability to “discover” the right amount of Darvan for your clay mix and water supply. And the ability to recognize what is wrong with a slurry that is not working (too much or little water, too much or little deflocculant).

This is how much casting slip 10,000 grams of powder makes

There are 8.8 liters of slip in this 2 imperial gallon bucket. The cone 10 stoneware slurry was propeller mixed in a larger bucket. First I stirred about 3/4 of the projected 44g of Darvan into 4000g of water. Then I dumped in 10,000g of the powder (shaken in a plastic bag) and let it sit to slake as much as possible. Then I used a high-energy propeller mixer, and to finish, trickled in extra Darvan until the rheology was right. The slip itself thus totals 14 kg (31 lb) and has a specific gravity of ~1.75. It has sat overnight and formed a film on the top, but has not settled (indicating that it likely is not over deflocculated). The casting process enables even a hobbyist to make his own custom recipes and tune them over time. Would you like to develop a recipe? We could use a group account at Insight-live.com to work together on it.

A slip cast bowl just removed from its plaster mold

With a simple open shape like this a thin wall (2-3mm) bowl can be cast in minutes and removed from the mold in minutes more. No other method can produce such thin and even ware with this kind of ease.

Talc:Ball Clay bodies have incredible casting properties

This bowl is 13cm across yet has a wall thickness of less than 2mm and weighs only 101g! It released from the mold with no problems and dried perfectly round. But it has a key advantage over stonewares and porcelains: When this is fired at cone 04-06 it will stay round!

Confirming slip viscosity using a paint-measuring device

A Ford Cup is being using to measure the viscosity of a casting clip. These are available at paint supply stores. This is a #4 (4.25mm opening), it holds 100ml and drains water in 10 seconds. This casting slip has a specific gravity of 1.79. Having made it many times, our experience indicates 40-seconds as a drain target (after high energy mixing). In production situations, the seconds-value this test produces enables an audit-trail for quality control and problem solving. When first mixing a slurry, under-deflocculating and eye-balling the viscosity is typical, during that period the slurry gels while draining and Ford cup measurements are not valid. When the mixing process has been perfected and viscosity stabilized the Ford Cup becomes practical.

Don’t overthink this type of measurement or the type of cup or opening size (you can even make your own cup). You must still determine the optimal flow rate based on experience with your process. This technique is more about maintaining to ongoing adherence to a standard you define.

Over deflocculated vs. under deflocculated ceramic slurry

The slip on the right has way too much Darvan deflocculant. Because the new recipe substitutes a large-particle kaolin for the original fine-particled material, it only requires about half the amount of Darvan. Underestimating that fact, I put in three-quarters of the amount. The over-deflocculated slurry cast too thin, is not releasing from the mold (therefore cracking) and the surface is dusty and grainy even though the clay is still very damp. On my second attempt I under-supplied the Darvan. That slurry gelled, did not drain well at all and it cast too thick. On the third attempt I hit the jackpot! Not only does it have 1.8 specific gravity (SG), but the slurry flowed really well, cast quickly, drained perfectly and the piece released from the mold in five minutes. Interestingly, on a fourth mix I made an error, putting in too much water, getting 1.6SG. The casting behavior was similar to the over-deflocculated slip (even though the Darvan content was much lower). A good casting slip is a combination of a good recipe, the right SG and the correct level of deflocculation.

New Zealand kaolin based slip casts at 1mm thickness. How?

This is Polar Ice casting, a New Zealand Halloysite based cone 6 translucent porcelain. The base body recipe would never have enough plastic strength to pull itself from this mold without tearing. But the addition of 1% Veegum gives it amazing strength. This dried cast bowl measures 130mm in diameter and 85mm deep, it only weighs 89 gm! The slip was in the mold for only 1 minute before pour-out. Of course, there is a price to pay for adding the Veegum: Increased casting time and more difficult deflocculation. Regular bentonite can be used in most bodies, but for super-whites like this, Veegum (or equivalent) is the choice. Testing is needed to determine what percentage gives the needed strength yet does not increase the casting time too much. The polar ice information page at plainsmanclays.com has very good information, under the heading “Casting Recipe”, about the challenges and trade-offs of using this kaolin in casting bodies.

Crawling glaze on slip cast ware is common

This cone 6 white glaze is crawling on the inside and outside of a thin-walled cast piece. This happened because the thick glaze application took a long time to dry, this extended period, coupled with the ability of the thicker glaze layer to assert its shrinkage, compromised the fragile bond between dried glaze and fairly smooth body. There are several measures that can be taken to solve this problem. The ware could be heated before glazing, the glaze applied thinner, or glazing the inside and outside could be done as separate operations (with a drying period between).

The casting slip did not drain well when pour this mug

The slurry contained insufficient Davan deflocculant. During casting it gelled excessively in the mold. Even shaking the piece while pouring out the slip was not enough to loosen it up and get a good drain. If slurry rheology is not right the quality of ware produced is affected like this.

My first 3D-printed handle case mold

These molds were my first effort using 3D design and 3D printing. They served well for making hundreds of pieces. They were part of a 2019 casting-jiggering project to reproduce a Medalta Potteries 1966 mug. Even with a thick handle like this and slip made using kaolin and ball clay not intended for casting, the molds split in less than an hour when casting solid (and in half an hour when casting hollow). As a parting agent on the 3D-printed surface I used Murphy's Oil Soap. The sidewalls had a draft of about 5 degrees and the handle cross-section was round rather than oval, so the plaster molds released without corner breaking.

This mold had spares - they did not provide a big enough reservoir and I later switched to using 3D printed spouts. These also had plaster matches (I later found that no matches worked better on this size and shape of mold). I went through multiple more versions perfecting the process to enable making handles of more difficult shapes and finally using 3D printed natches.

A heavily grogged casting body still casts with a smooth surface!

20% 20-40 mesh grog was added to a Pyrax/Kaolin thermal shock body. While the insides of the pieces have a very rough surface, the outsides are smooth! Grogged casting slips have issues with the particle settling during storage and casting, however in this body the grog suspends long enough for a 15 minute casting time (and it easily mixes back in after storage). Pieces can be put into the kiln wet-out-of-the-mold and fast-heated to 250F and they do not crack.

A 3D-printed spout enables a flared rim on cast ware

It was glued down using the casting slip itself (it stuck in seconds). About ten minutes after draining a fettling knife was run around the inside, then it detached easily. The overhung lip produced imparts structural strength that resists warping, for drying and firing, to the thin walled piece. This spout has advantages over the traditional "spare" built in to the upper part of a mold. It enables a one-piece mold. The lip can be more overhung. Draining is cleaner and easier. Molds are lighter. Extraction can be done sooner and it is easier. The spout does not absorb so there is less scrap. The degree of overhang is adjustable by simply printing new spouts.

Casting plates, is it practical?

No. Because you will face a whole array of problems, not the least of which is the tearing shown here. This is the first, poor mold release, or more correct, impossible mold release! Plates will be too thin-walled. If you cast them longer wall thickness will be uneven. Edges will crack like this (because of poor plasticity). They will warp during drying. They will lack dry strength for handling. They will warp during firing. You won't be able to get a good rim. You won't be able to cast a foot ring without an indent showing on the inside. Note here that another issue is at play: The clay is either not plastic enough to cut cleanly at the rim, without tearing. Or, it is being cut too late or with a dull knife. These tears provide places for cracks to initiate. Plates are much better made using the jiggering, ram pressing or dust pressing processes. Or by throwing them on plaster batts.

Bottle calibration mold demos some casting process improvements

Three slip casting methods are examples of the flexibility potters have compared to manufacturers (especially for small molds like this).
1) 3D-printed locks are stuck on to hold the mold halves together. The casting slip itself adheres them. Dipping the flat surfaces and attaching them takes seconds. By using this method the mold halves can be aligned accurately.
2) The 3D-printed pouring spout is also attached using the slip (it also helps hold the mold halves together) and acts as a reservoir and pour-out indicator.
3) There are no notches or natches (the halves were poured into disposable 3D printed PLA masters - and mate perfectly and align easily).
These three factors simplify mold design, reduce mold size, improve the fit of parts and simplify pouring, demolding and cleanup.

Here is how vigorously a deflocculated ceramic slurry should be mixed

A video of the kind of agitation needed from a propeller mixer to get the best properties out of a deflocculated slurry. This is Plainsman Polar Ice mixing in a 5-gallon pail. Although it is quite plastic compared to industrial casting slips, it has a specific gravity of 1.76, is very fluid and casts well in the hands of a potter. These properties are a product of, not just the recipe, but the mixer and its ability to put high energy into the slurry.

3D-printing artifacts under an amber glaze on light and dark bodies

The dark one is M370C with 10% added raw umber. The other is M370C. Both are glazed using GA6-B Alberta Slip amber transparent - the color difference is entirely a product of the difference in color of the fired clay body. The wood-grain texture is the artifact of 3D-printing the case mold flat rather than upright.

Inbound Photo Links

Why mid-fire Grolleg porcelain is ideal for both throwing and casting

Draining has started turning this narrow-necked bottle inside out

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