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A building restoration specialist is developing an engobe recipe to apply to already-fired solid bricks (to match the facade of a historic building). This is challenging because engobes are normally applied at leather-hard stage (thus dry and fire shrinking with the body). Of course, the fired color, character and weather durability are most important; but, to be usable, this recipe needed other properties: Good adherence to the surface and ability to stay adhered during firing (thus low drying shrinkage and low firing shrinkage). That is the reason for the CMC gum, calcined versions of the clays, the grog, and the wollastonite.
Red Art - 10
Calcined RedArt - 10
Calcined kaolin - 10
Calcined ball clay - 10
Calcined Newman Red fireclay - 5
Ferro Frit 3110 - 25
Grog - 30
Wollastonite - 10
CMC Gum - 1
The percentage of frit is very important; it can only be determined by testing. Too much and the firing shrinkage will be excessive (it will flake off). Too little and the durability and weather resistance won't be sufficient. If a sufficiently durable surface cannot be achieved, then multiple layers that transition from hard surface to the brick substrate might be another approach.
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