... and this has just added to my impression that terracotta is pretty much whatever you want it to be
And -- even more confusingly - sometimes the term is used based more on the nature of the object made
(rather than the body used or firing temperature).
Vessels and other objects that are or might be made on a wheel from the same material are called earthenware pottery; the
choice of term depends on the type of object rather than the material or firing technique. Unglazed pieces, and those made
for building construction and industry, are also more likely to be referred to as terracotta, whereas tableware and other vessels
are called earthenware (though sometimes terracotta if unglazed), or by a more precise term such as faience.
Looking for clarity I consulted Architectural Ceramics, David Hamiltion.
The terms 'terracotta' and 'faience' are generally understood to mean, respectively, unglazed and
glazed earthenware. Contrary to the commonly held belief, 'terracotta' does not only refer to red-
firing unglazed clay but may be applied to any unglazed clay.
At least one factory creates a terminological dilemma by calling all hand-pressed facings, whether
glazed or unglazed, 'terracotta', and all cast or mechanically made pieces 'faience'.
PS The traditional Spanish ‘botijo’ seems to come in all sorts of colours. In my very limited experience
they are made with quite thick walls, which I assume is a functional decision.
... this page includes an experiment keeping water cool in an oven, and a ref to an old technical paper.
After 7 hours in an oven at 39C the water inside the cooler was at 24C. Wow!