|Monthly Tech-Tip |
These lines plot the firing shrinkages for three versions of L3685Z2 engobe. Notice the terra cotta body I want to match (red line) and the black engobe (green line) do not cross anywhere. That means there is no temperature at which they fit each other (the engobe always has 2% or more firing shrinkage). Notice the L4170B terra cotta fits the white version, Z2, at 2150F (red line crosses blue line, but the body is over-fired by that point). For a fit at my preferred 2000F (cone 02) I need the Z4 engobe to shrink 2% more (a 3% addition of frit 3110 will do that). What about the black Z4? That is the opposite situation, it already contains 5% frit, removing that will drop that green line about 3%, hitting the red line at 2000F (and following it all the way down past 1950 into the cone 04 range). I ignored all of this and used the Z2 white on L4170B, L210 and L215. It looked good on most pieces, but sure enough, it did crack around abrupt contours on some. Of course, this does not assume a thermal-expansion-match of body and engobe.
Engobes can be incredibly opaque. This very thin layer of L3685Z2 completely covers these terra cottas (L210 and L215). Its color is whiter than paper! Using our G1916Q and G3879 clear overglazes, ware can appear as white as porcelain! But notice there are tiny cracks in the white on the edges of contours (most noticeable on the left sample). CMC gum was added to this engobe, enabling applying it to the bisque. It appeared to work well, but during firing the engobe shrank 2%, putting it under tension (the body had already shrunk during its bisque). If it were applied to the leather hard ware that would not fix the problem. Why? Because the engobe has a 2% lower firing shrinkage than these bodies. That would put it under compression, looking for opportunities to flake off at edges (e.g. rims of mugs). How to fix this? The engobe needs about 3% Ferro Frit 3110 to raise its firing shrinkage by 2%. And, to only be applied to leather hard ware.
The term Terra Cotta can refer to a process or a kind of clay. Terra cotta clays are high in iron and available almost everywhere. While they vitrify at low temperatures, they are typically fired much lower than that and covered with colorful glazes.