According to the National Starch & Chemical Company, the glue used in envelopes is made with corn starch and tapioca.

"Following is the information regarding gluten in food and pharmaceutical starches which we supply and the subject of Celiac Disease".

The food and pharmaceutical starch products we supply are refined from dent corn, waxy corn, or tapioca. Celiac disease is an allergic reaction to certain amino acids found in the protein of wheat and rye flours (wheat gluten).  Wheat and rye flours have a protein content ranging from 5% to 14%.  The food starches that we supply typically have a protein content of less than 0.5%.  Most of the protein content of corn and tapioca is removed in the refining process and is sold as a by-product. 

In addition to the low amount of protein in corn and tapioca starch, the types of amino acids are not identical with those found in wheat or rye flours.  Therefore, they are not associated with Celiac disease."

National Starch & Chemical Company
Food Products Division
10 Finderne Avenue
P.O. Box 6500
Bridgewater, NJ 08807-0500
908-685-5000