Hi Michael,
Engobe is a loan word in German, from English, via French. The terms ‘slip’ and ‘engobe’ are often mistakenly used interchangeably in pottery, but there is a significant difference. Having mixed many a batch of both, I’m all too familiar with the differences.
Generally, a slip is just a mixture of clay and water, thinned to a consistency of thick cream. It may or may not have oxides added to it for colorants. It can be used for joining pieces of leather hard clay together, or for a wide variety of decorative techniques.
Since it has a high percentage of clay (slip is just the same clay body used in making the pot, blended with a lot of water), it can only be applied to a pot when it’s relatively wet, in the soft leather hard stage. (Brits will call this cheese hard). Due to the shrinkage caused by the high percentage of clay, if applied when the pot is too dried, it will just shiver or peel off when dried, and possibly crack the pot.
An engobe plays a similar role, i.e. a brushable mixture of cream-like consistency, but is used only for decorative purposes. It is formulated using much less clay, and substituting instead higher percentages of silicas and fluxes, which results in a far lower shrinkage rate. Thus, it does not have to be applied at the leather hard stage, and can be applied at any of the stages: leatherhard, bone dry, or even after the pot has been bisqued. Since it shrinks at a different rate, it cannot be used for joining pieces of clay.
An engobe is somewhere between a slip and a glaze. With the higher amount of flux, the bonding to the pot is actually a result of the silicas (lowered by the fluxes) fusing to the body, whereas slip bonds are simply formed by the interlocking clay platelets.
Most of the commercially prepared underglazes used should be considered engobes since they’re applied at any of the stages.
Hope that helps.