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Making Better Clay Slip

Water and clay are pulled into the top and bottom of the spinning head, and expelled thru slots in the sides. The centrifugal force creates high shear forces that tear apart the clay lumps.


We rented a HydroMax food-grade mixer from Silverson Company. It uses special stainless steel heads driven at high RPM by a variable speed motor.

With it we were able to make a smooth mix of evenly dispersed clay particles -- perfect slip for plasters and straw-clay walls.


Minnesota scientist Douglas Piltingsrud is an expert in coatings and mixing technology. He directed our investigation of improving clay slip and earthen plaster recipes.


Jeremy thickens up a mix under Douglas's tutelage.

Slip for straw-clay walls is made with ordinary earth, albeit high clay-content earth. We have ours delivered by a local landscape contractor, who shreds it into easy-to-use granules with a topsoil shredding machine.

For the finer finish coats of plasters we use commercially bagged clays and finer, lighter-colored sands. We used a mixing head with smaller slots (above) for these lighter, powdered materials and other admixture ingredients.

The HydroMax's speed controller (above, at left) allows fine control over the mixing process. It also adapts the 220 volt 3-phase motor to 120 volt single-phase current.


Adding casein to water
Casein is a milk product which, when used in conjunction with lime, improves the moisture performance of interior plasters. We purchase food-grade casein as dry powder from a dairy lab.


Adding lime putty to the mix
Lime, used in buildings for millenia, is made by heating limestone. When added to clay mixes in the proper proportions, it increases the weather resistance and durability of plasters.


Colored pigments
The color in our plasters is integral. Here we add yellow and red iron oxide powders -- pigments that potters use in their glazes -- to white kaolin clay to achieve a warm golden interior plaster.

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