• Materials for Ceramics

OptiKast Kaolin

Alternate Names: OptiCast

Description: Large Particle Size Kaolin

Notes

optiKast is a coarse-grained kaolin used primarily in ceramic casting applications. It has a very fast cast and exceptional deflocculation characteristics. Thus optiKasT is preferred in pressure casting systems.

Velvacast kaolin is no longer available, this is the recommended substitute. One concern with Velvacast was its dark fired color (compared to other common plastic kaolins). Opticast has higher iron and TiO2 contents, this will impact whiteness and certainly translucency in porcelains. We have done fired shrinkage, porosity, dry performance and plasticity comparison tests between these two materials Oct 2011 and they do appear very similar, Opticast is a little more plastic than Velvacast and does not appear to fire darker at cone 8 and 10.

Related Information

This cone 6 whiteware casts super-fast - learn why

This is L4807 cone 6 whiteware casting slip. It casts at incredible speed. It is an example of a body that a supplier would not make but as a potter you can make it yourself.

Typical slips made from ball clay and talc (or mixes of common feldspars, clays and quartz) would take 20 minutes in this mold to get the needed thickness. But this one pours out in 12 minutes. We are used to extracting pieces from molds in 30-60 minutes, with this clay I can extract in 10 minutes (assuming, of course, the slip is properly deflocculated and molds are in good condition). But in this case the secret is the recipe: It employs OptiKast Kaolin and KT#1-4 ball clay, both are large particled materials targeted at slip casting. But there is a down-side, read more about it in the L4807 recipe information.

How a kaolin and ball clay compare in a dry performance test

These are DFAC drying performance disks of a large-particle kaolin (OptiKast) and a ball clay (Plainsman A2). This test reveals a clay's response to uneven drying (these disks are dried with the center portion covered). The kaolin feels smoother yet its ultimate particles are ten to one hundred times bigger than a typical ball clay. Thus it shrinks much less. The ball clay has dramatically lower water permeability, water from the center protected portion resists migration to the outer edge during drying. When the inner section finally dried the outer was already rigid so it split the disk in two and pulled all the edge cracks. Most ball clays shrink more and crack worse than this (cracks concentric to the center also appear). So why use ball clay? This kaolin is so lacking in plasticity it was barely possible to even make this disk. And it is so weak that it can easily break just by handling it. Still, it is useful to make casting bodies. But the ball clay, when used as a percentage of a body mix, can produce highly plastic bodies than can be dried without trouble if done evenly.

First mug in my newly created mold

This test mold is thin-walled yet I can cast three thick-walled mugs in three hours. This clay is L2596G, a buff burning cone 10 stoneware - the mug on the lower right has been fired to cone 10 oxidation. Achieving 4-5mm thick walls is not a problem if the casting slip employs a large particle kaolin intended for this purpose (e.g. OptiKast). And it is deflocculated properly.

The flared lip works as expected, keeping the rim nice and round. No cracks have appeared at handle joins, even for pieces left in the mold overnight. The mold halves mate with each other very well and the seam is easy to remove. The seam on the base is an issue - I have to be careful to line up the halves well before clamping the mold strap - this is a warning for accuracy during the mold production stage. And the possible motive for a three-piece mold if I get more serious about this piece.

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