| Why glasses sometimes get a white
film on them.
The glasses probably developed what is often called etching.
Etching is a permanent chemical alteration of the glass surface. Etched
glasses appear cloudy or hazy. Once etched, there is no practical method
of restoring the glassware
to its' original condition.
Table glassware
is comprised of what is known as "soda-lime" glass. The major
ingredients are sand, soda ash and lime. Etching is a process whereby
the metal ions are removed from the glass surface, leaving the silica
exposed. Libbey's glass composition has been formulated to offer superior
resistance to etching.
The etching process is normally a very slow one. Many factors can be
involved, and the etching is usually due to a combination of those factors.
All glassware
has some tendency to etch with age, as the metal ions in the glass are
slowly solubilized with repeated washings.
Etching is more likely to occur if the water is soft. Natural or artificially
softened water promotes the etching process. Soft water tends to dissolve
more of the alkaline (sodium) metal ions faster than hard water. Consequently,
the exposed silica will appear sooner. Often the early stages of attack
of the glassware
are typified by the appearance of numerous tiny scratches, which progressively
increase until the etch film is evident.
Hard water can cause filming of glassware,
due to the presence of calcium or magnesium ions and/or bicarbonate alkalinity.
However, that type of film is readily removed through acid treatment of
the glassware.
The rule of thumb is that if the cloudy film cannot be removed from the
glass by acid or concentrated bleach (which will dissolve food soil),
then the film is probably etch. If a hard water film or scale on the glass
is allowed to remain there for an extended period of time, it can have
an adverse effect on the glass surface.
Proper wash temperatures are important. Wash temperatures above 150°
F. will accelerate etching. At 160° F. substantial damage can occur.
Very little surface damage should be noted at temperatures up to 140°
F.
Detergent concentration should be kept within the manufacturers' recommended
range. Too low a concentration can result in incomplete removal of food
soil from the glass surface. In those instances where small amounts of
soil are left on the surface, a physical phenomena known as occlusion
can take place. Occlusion means that chemicals used in the washing process
may be held by the soil and, therefore, remain in contact with the glass
for long periods of time. That can be a factor in etching problems.
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