Synthetic glues like Elmer's are made of polyvinyl acetate (PVA) emulsions. The word emulsion refers to the fact that the PVA particles have been emulsified or suspended in water. The thinner the glue, the more water it has in it.
In addition to water and PVA, synthetic glue can also contain ingredients like ethanol, acetone, and amyl acetate. Ethanol slows down the drying time, acetone speeds up the drying time, and amyl acetate slows down the evaporation of the glue. However, some natural glues – called hide glues or protein adhesives – are made from animal proteins or gelatin. This is why people often speak of dead horses being sent to the glue factory.
Humans have been experimenting with animal-based adhesives for thousands of years, and the world's oldest glue on record has been carbon dated as being over 8,000 years old. Archaeologists discovered the glue in Israel in a cave near the Dead Sea. The glue was made from collagen, an animal protein, and it was used as a water-sealant, an adhesive and a protectant.
Four thousand years ago, ancient Egyptians used similar collagen-based glues in furniture construction, and about 1,500 years ago, Native American people used collagen-based glues while making bows for hunting and batlle.