How It Works

What’s in the glue that seals envelopes?

Assuming you are talking about the glue that needs moisture to seal the envelope. This type of glue is made from gum arabic, which is a product of the hardened sap from two different kinds of acacia tree, which can be found in west Africa and northwestern India. It is edible for humans and has E number E414. As well as being useful for sealing envelopes it is also a staple ingredient in certain sweets such as M&Ms, marshmallows and gumdrops. Non-edible applications also include the binding of watercolour paint and an important ingredient of shoe polish. Pretty useful stuff.
Author: Rik Sargent, Science Museum