|Monthly Tech-Tip |
Alternate Names: Macaloid
Description: Glaze Suspender, Hectorite
Bentone MA is a highly refined hectorite clay that can be used as a body plasticizer and a glaze suspension agent. At one time it was known as Macaloid, and many resellers in the pottery and hobby supply industry still refer to is by this name.
Bentone MA suspends glaze slurries and hardens the dried glazes while not extending the drying time as much as CMC gum (or VeeGum T). Thus it is common to use it and CMC together to control drying time, suspension and glaze hardness. It is valued for its long-time suspending properties which helps prevent uneven settling of material particles which have differing specific gravities (i.e. stain particles tend to be much heavier and settle first).
Bentone is much lower in iron than bentonite and thus is useful as a plasticizer in very white burning porcelain bodies. It increases water requirements somewhat, but not as much as VeeGum T (although it imparts slightly less plasticity).
To get maximum benefit from this material, it should be mixed thoroughly into the water with a power blunger (for hours if possible) before adding the other dry ingredients.
C.E.C.: 78 me/100 gm
Principle Exchange Ion: Na
pH (5% solids slurry): 8.5
Hectorite Content: 90%
Minus 50 mesh: 90%
Minus 325 mesh: 99.5%
Surface Area: 600 sq. meter/gram
Bulk Density (not compacted): 0.24-0.37 g/cc
Bentone grades HC, CT, EW are used as rheology modifiers in glazes.
When formulating a white throwing porcelain that employs a white expensive plasticizer (like Bentone or Veegum) the optimal range of percentages can be surprisingly narrow (I am assuming at least 40% kaolin is present). The trimming behavior is one indicator. When there is insufficient plasticizer the tool will chatter (of course in extreme cases edges will tear). Smoothing the corners after trimming (using your finger) will also give you an indication. If there is too much plasticizer, the material will ball up under your finger, if there is insufficient it will not smooth out well. The percentage can be critical: 0.5% too high and the drying shrinkage could sky rocket, 0.5% too low and lips can split at the rim during throwing.
Bentone (AKA Macaloid) is a super plastic additive used to modify rheolgy in many consumer products. It is made by refining Hectorite. It is very difficult to mix pure Bentone with water, it is just so sticky and the water content is so high, it takes a week to dry a sample and it cracks into pieces during drying. I am studying five different grades for use as a plasticizer in premium porcelains. I am interested in how they stack up against the king: VeeGum T (both in price and performance). The first step was to fire square tiles of the powder on small porcelain tiles at cone 6 to compare the iron content. Three sintered into a solid mass, shrinking to about half the size. The CT grade is the natural, untreated Hectorite clay (accounting for its darker color), the processing to purify the material obviously increases its affinity for water, shrinkage and fired maturity.
The glass on the small tile at the right drained out from this specimen of pure Bentone MA as it was fired at cone 6. The remaining skeleton is on the left.
The Veegum is dense and white, but not melting. The Mineral Colloid fires like a typical raw bentonite (dark brown, high soluble salts and beginning to melt). The Gelwhite is completely melted and foamed.
Bentone (A.K.A. Macaloid MA) is a very plastic highly refined hectorite clay. This specimen has been mixed as a slurry, then dewatered until plastic on a plaster slab (it is very resistant to giving up its water). The plastic material has a very high water content, is exceptionally sticky and took many days to dry from the plastic stage. It shrinks 30% or more from plastic to fired and burns pure white at cone 6 (it can withstand higher temperatures). It burns whiter than similar materials from other manufacturers.
A clay of incredibly small particle size. It has the highest plasticity of any known clay and acts as a suspending and gelling in slurries.
Bentonite can make a clay body instantly plastic, only 2-3% can have a big effect. It also suspends slurries so they don't settle out and slows down drying.
An uncommon fine grained highly plastic clay mineral related to bentonite. Named after Hector, CA. F
A clay mineral of extremely small particle size and high plasticity. Raw bentonite is generally a pa
Clays that are not kaolins, ball clays or bentonites. For example, stoneware clays are mixtures of all of the above plus quartz, feldspar, mica and other minerals. There are also many clays that have high plasticity like bentonite but are much different mineralogically.
Bentonte MA Data Sheet
|By Tony Hansen|
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