Over many years of collecting vintage Coca-Cola items, on several occasions I have had a glass object such as a fountain glass (or in today's instance a Coca-Cola chewing gum jar, that I display in a wood and glass case) that has become cloudy or hazy in appearance.
This cloudiness is not to be confused with glass that simply has mineral deposits on the inside or outside from containing or sitting in water for a long period of time. The cloudiness I am describing is mostly on the outside surface of the object and is oily or slimy feeling when touched.
This oily film is called "Crizzling". It is the result of chemical instability in the make up of the glass caused by an imbalance in the ingredients that make up a particular batch of glass during the production process. (such as too much alkali or a lack of lime stabilizer).
Alkali, is a soluble salt consisting mainly of potassium carbonate or sodium carbonate and accounts for approximately 15-20 percent of the batch. Alkali is considered a flux, which reduces the melting point of glass's main ingredient silica (sand). Lime is a calcined limestone, which is added to the batch in small quantities to give the mixture stability.
When the glass is chemically unstable, it become susceptible to atmospheric moisture -especially when it is enclosed in a display case (especially a wood case, because wood tends hold moisture for a long time). The resulting instability in the glass - combined with atmospheric moisture - causes tiny droplets or fine crystals of alkali to leech from the glass if there is relative humidity above 55% or below 40%.
This presence of alkali on the glass is what gives the glass surface a cloudy or hazy appearance. Crizzling can be slowed or halted, but it cannot be reversed. Washing the object with lukewarm tap water and a mild (non-ionic) detergent, the rinsing with de-ionized or distilled water will remove alkali from the surface and bring the glass back to its original appearance. The older the object is and the longer the object has been subjected to crizzling increases the chances for permanent damage and haziness.
This is exactly what has happened to my chewing gum jar. But hopefully, regular cleaning and attention will keep it from becoming damaged.