Wikipedia
Wheat paste (also known as flour paste, or simply paste) is a gel or liquid adhesive made from wheat flour or starch and water. It has been used since antiquity for various arts and crafts such as book binding, découpage, collage, papier-mâché, and adhering paper posters and notices to walls. Closely resembling wallpaper paste, a crude wheat flour paste can be made by mixing roughly equal portions of flour and water and heating until the mixture thickens.
A critical difference among wheat pastes is the division between those made from flour and those made from starch. Vegetable flours contain both gluten and starch. Over time the gluten in a flour paste cross-links, making it very difficult to release the adhesive. Using only starch, a fine quality, fully reversible paste can be produced. The latter is the standard adhesive for paper conservation.
Besides wheat, other vegetables also are processed into flours and starches from which pastes can be made: characteristics (e.g. strength, reversibility) vary with the plant species, manufacturer's processing, and recipe of the end-user.