Color matching restoration project: Engobe on fired brick

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.

Related Pictures

Engobe adjustment parameters

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Here is why you don't want an engobe to melt

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Too much frit in an engobe and it will lose opacity and whiteness. The white slip on the left is an adjustment to the popular "Fish Sauce" slip recipe (L3685A: 8% Frit 3110 replaces 8% Pyrax to make it harder and fire-bond to the body better). The one on the right, L3685C, has 15% frit. Although applied at the same thickness, it is becoming translucent, moving it into glaze territory. That means it will have a far higher firing shrinkage than the body (a common cause of shivering at lips and contour changes). This slip is basically a very plastic white body. Since white burning slips are made from refractory materials they are not nearly as vitreous as red ones, at low fire they need help to mature and a frit is the natural answer. With the right amount of frit the fired shrinkage of body and slip can be matched and the slip will be opaque. This underscores the need to tune the maturity of an engobe to the body and temperature. Although zircon could be added to the one on the right to opacify and whiten it, that would not fix the mismatch in fired shrinkage between it and the body. And it would increase the price.

How to test if an engobe fits a clay body

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This is part of a project to fit an engobe (slip) onto a terra cotta at cone 02 using the EBCT test.
Left: On drying the red body curls the bi-clay strip toward itself, but on firing it goes the other way!
Right: SHAB test bars of the white slip and red body enable comparing their drying and firing shrinkages.
Center back: A mug with the white engobe and a transparent overglaze. The slip is going translucent under the glaze because it is too vitreous. Its higher fired shrinkage curls the bi-clay bars toward itself. Reducing the frit will reduce the firing shrinkage and make it more opaque (because it will melt less).
Front: A different, more vitreous red body (Zero3 stoneware) fits the slip better (the strips dry and fire straight).

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The raw materials research hub for the Division of Ceramic Art at Alfred University. Their mission is to support artistic research via ceramic materials. Information, publications and social posts on materials, safety, recipes, procedures, etc.

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