Adhesives (Gums) on Stamps. 
In philately, gum is the substance applied to the back of a stamp to enable it to adhere to a letter or other mailed item. The term is generic, and applies to both the traditional types such as gum Arabic and to synthetic modern formulations. Gum is often referred to as an adhesive; a substance which causes adhesion.
There have been four adhesives that have found use on postage stamps.
Gum Arabic is a direct exudate from the acacia tree found in various tropical areas. Its composition is a complex mixture of Calcium, Magnesium, and Potassium salts of Arabic acid. Used as a stabilizer and thickener in foods, its main nonfood uses are in the formulation of inks and adhesives. It is highly soluble in water.
Dextrin, also called British gum, is a water-soluble, gummy substance composed of small polysaccharides (carbohydrates), prepared from food grain corn starch. Dextrin applied to the backs of postage stamps has a shiny gum, usually with a yellowish tinge, that tends to deteriorate with age. This is clearly shown in the scanning electron photomicrograph (SEM) shown below. The photo (500X magnification) shows a cracked adhesive layer and the underlying layers of cellulose of the stamp paper.(Figure 1).
Polyvinyl Alcohol (PVA), is a colorless, water soluble resin used chiefly as an adhesive and as a sizer in the manufacture of textiles, paper, and plastics. Since it does not absorb moisture, it is ideal for tropical use and for such applications as stamp booklets, which may sit in outdoor vending machines. It is a white gum of dull or matte appearance. The gum exhibits some texture or grain, as is shown in this SEM photograph at 1000X magnification. (Figure 2).
Pressure–Sensitive Adhesive is a rubber base adhesive that does not require moistening to adhere to paper. They are usually issued on removable backing paper.Formulations used in the adhesives on current postage stamps can vary in order to impart properties to the adhesives. For example, blends of PVA and polyvinyl acetate, as well as blends of dextrin and polyvinyl acetate, have been used.
For the stamps of Ireland, all four of these adhesives have been used at various times. Prior to Decimalisation in 1971, Gum Arabic was used on all Irish issues., but from 1968 PVA was gradually introduced to replace Gum Arabic. The PVA after 1970 appears both clear and matte and often quite transparent on the one hand, and with yellowish additive in varying concentrations on the other hand. Thus the PVA is described as being with both dull gum and shiny gum, such as the 2 1/2p stamps of the decimal watermarked issue (Early Irish Art, 1968-69). An Post introduced self adhesive stamps for the first time on October 31, 1991. The pressure sensitive adhesive has been especially formulated to allow stamps to be washed off paper, without damage, after submersion in warm water. It has been observed that current self adhesive stamps are much more difficult to remove from envelopes. No doubt the formulation has changed over the years.
(Extract by Dr. John Sharkey) 9/27/2018