Lickable glue for handmade envelopes (gum arabic)

Question

I am looking for a way to put down a strip of adhesive on the tab of handmade envelopes. I would like to have the adhesive remain dry on the envelope and only become tacky when it is licked/moistened-- just like on regular envelopes.

This has historically been done with gum arabic, but I can't find a "recipe" to properly mix the gum arabic for this application. Moreover, gum arabic seems to come in different forms (powder, grains, or liquid) so it's not obvious how to get started with using it as a lickable adhesive. It seems that gum arabic is mostly used as a paint additive these days.

Does anyone know how to make or buy this type of adhesive and have instructions for its proper application?

**** UPDATE ****

I (mostly) followed the instructions that were kindly provided by @Nothingismagick.

After finding food-grade gum arabic and yellow dextrin on Amazon and clove oil at Whole Foods:

I followed the instructions in the answer. Everything went OK until I tried to dissolve the yellow dextrin. 25g of dextrin and 50ml of water produced a mixture that was hard to dissolve. I got impatient and added a LOT more water to the dextrin.

The end result was a solution that was too thin. It just soaked and stained the paper without producing a lickable layer. I put the jar on a radiator without the lid for a few days and this reduced the volume by half. The solution did get thicker-- like maple syrup. This works much better, but I think I should reduce it even more. Here's the final result.

I will try again and be more careful/patient when mixing in the yellow dextrin and the water!

Answer 1

tldr; Classical envelope glue is made with gum arabic and yellow dextrin. Both gum arabic and yellow dextrin are dissolved in hot water. To prevent rot, refine with copper ions and a drop of clove oil. Apply to paper.

100 ml distilled water
10 g gum arabic (powdered)
25 g yellow dextrin (or 20 g tapioca starch + 10 g sugar)
1 drop clove oil

You can purchase gum arabic online or at better art supply stores - take whatever form you can get. If it is in crystal form (semi-irregular rock-salt size) crush it with a mortar and pestle.) Use only distilled water. A copper pot is not absolutely necessary, but copper ions are antimicrobial and antibiotic. Clove oil can be replaced with peppermint oil or left out if you do not plan to store it.

Crush the gum arabic with a mortar and pestle and then soak overnight in a copper pot with 50 ml of the water. In a glass bowl, sift the dextrin (or starch and sugar) into the remaining 50ml of water and mix until the water is cloudy and no clumps remain. Heat the water and gum arabic mixture, but do not bring to a boil! Add the drop of oil and gently whisk in the dextrin with a stainless steel hand whisk. Continue whisking until the solution gels. Remove from heat and pour into a clean and dry mason jar.

When cooled, brush thinly onto your paper envelope and let dry (or use a blow drier to speed the drying). If it gets too thick, thin with distilled water. Keep cool and in a dark place. Do not freeze, use within 6 months.

Bonus: To make yellow dextrin, spread 500g of tapioca starch out on a dry and ungreased cookie sheet and put in an oven at 200 °C. Stir it with a metal spatula every 10 minutes until it has all turned a golden yellow colour. Remove from oven and let it cool down, then put in an airtight container.