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 within a short time, the item stiffens and shrinks slightly and can be removed from the mold. This process is flexible, capable of producing fine delicate porcelain items yet heavy sanitary ware. The casting process is normally 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 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 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 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). Many potters use a dual-purpose body recipe (e.g. the casting version having 1% bentonite and the plastic version having 3-4% bentonite. The above being said, bodies can actually cast and release too fast, causing issues in production.

Because casting bodies are relatively non-plastic they have very low drying shrinkages compared to plastic ones, ware can be dried quickly with much less likelihood of cracking. Pieces often do not even need to be covered during drying. Separately cast, leather-hard sections can be effectively glued together with slip and the pieces can be dried without issue. That being said, non-plastic ware is often difficult to release from molds if walls are thin or shapes are complex. To deal with release problems it may be necessary to cast thicker, add a plasticizer to the body (e.g. 0.5-1% bentonite) to give it more leather hard strength and make sure molds are in good condition.

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 reaction to this is a body recipe that is more plastic than needed (usually by the addition of a small amount of bentonite, e.g. 0.5-1%).

3D printing technology has many applications to casting, especially for mold-making and tooling.

Related Information

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

To-the-brim the bucket holds 8.8 liters (2.43 Canadian gal, 1.9 US gal). The slip itself weighs 14 kg (30 lb). It has a specific gravity of between 1.75 and 1.8. The slurry was power-mixed in a larger bucket.

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).

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!

Measuring 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.

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.

The incredible utility of 3D printing master handle molds

These molds are 3D-printed from PLA filament. They are part of my 2019 year-long casting-jiggering project. A quick soaping, 164g water, 236g plaster and a fifteen minute set produced this plaster mold. It takes time to learn how to soap the masters properly to get optimum quality, but these molds seem to work well regardless. The two halves mate with a tiny amount of play, but it is easy to line them up perfectly (the play actually enables lateral movement that aids in releasing the handle). It is actually easier to cast handles solid rather than pour the slip out, they can be ready to apply in an hour after pouring. The ease of making these molds puts slip casting within much easier reach for potters and small companies.

3D-Printed master mold, handle mold made in it, first cast

The halves fit together well. Even with a thick handle like this, the mold splits for me in less than an hour when casting solid (and in half an hour when cast hollow). The cast handle is very sturdy and easy to cut and glue on to leather-hard mugs. As a parting-agent I use Murphy's Oil Soap on the 3D-printed PLA mold, this makes it fairly easy to extract the freshly-cast plaster molds (the sidewalls have a draft of about 5 degrees). The PLA mold is very durable so it is practical to use the end of a knife to get soap residue out of the recesses (when cleaning). If I do not remove the soap bubbles sufficiently before pouring in the plaster, corners are poor quality (so the mold produces a more visible seam on the cast handles). Notwithstanding that, the open plaster mold shown produces excellent quality handles.

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.

Optimimal casting slurry properties impossible without good mixing

A video of the kind of agitation you need from a power mixer to get the best deflocculated slurry properties. This is Plainsman Polar Ice mixing in a 5 gallon pail using my mixer. Although it has a specific gravity of 1.76, it is very fluid and yet casts really well. These properties are a product of, not just the recipe, but the mixer and its ability to put energy into the slurry.

Mini beer bottle demonstrates casting behavior, fired color, glaze appearance

The dark one is M370 with 10% added raw umber. The other is M370. Both are glazed using GA6-B Alberta Slip amber transparent. This glaze is the best functional super gloss recipe we have. The wood-grain texture is the artifact of 3D-printing the mold-mold, they are not detectable to the touch. And actually look great - like wood grain.

Beer bottle calibration mold demos some casting process improvements

This is another example of the flexibility potters have compared to manufacturers. These 3D-printed gizmos are stuck on to hold the mold halves together. The casting slip itself adheres them. Dipping the flat surfaces and attaching them takes seconds. The 3D printed pouring spout is likewise attached using the slip (it also helps hold the mold halves together). Another feature: There are no notches (the halves were poured into disposable 3D printed PLA masters - and mate perfectly). Initial use of a rubber band to hold them together was not ideal because realignment of the halves damaged the inside corners. By using this method the mold halves can be aligned accurately. These three things are great for potters. They simplify mold design and production, reduce mold size, improve the fit of parts and simplify pouring, demolding and cleanup,

Inbound Photo Links

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Draining has started turning this narrow-necked bottle inside out

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