Gesso

She primed her canvas with gesso before starting to paint.

1994, Timothy Noad, Patricia Seligman, The Illuminated Alphabet, The Quarto Group (Chartwell Books), 2017, page 27, The combination of leaf gold and gesso is almost miraculous. No photographic reproduction can adequately show the brilliant effect of raised gesso.

To make the gesso, you need a chalk and a glue. During the Renaissance, gesso was made with gypsum, which is calcium sulphate. Terra Alba, available from art suppliers today, is a natural gypsum that makes a bright white gesso.

2007, Robin Cormack, Icons, The British Museum Press, Harvard University Press, page 33, The idea was that this would serve as a binder for the layer of gesso or at least might help to prevent the painting from instantly cracking apart if the wood split at any time.