Staining or Browning of the Glass
Stains on glass surfaces of
window-tank slabs are caused by chemical reactions of oil
contaminants with mineral constituents in the glass. Changing the
oil is imperative; this will not remove the stain, but will arrest
the degradation. Chemical analysis of the drained oil should reveal
what caused the stain, so precautionary measures can be taken to
protect the other cell windows.
Browning is caused by changes in molecular structure of the glass
induced by radiation. Browning decreases the light transmittance; in
extreme cases, the glass may turn coffee brown or black, and may
even become opaque.
The addition of cesium oxide (CeO2) to the glass during
manufacture ameliorates this discoloration by effecting recovery of
the glass. The coloration process continues under irradiation, but is
almost totally reversed when the radiation is removed. Thus, cesium
oxide stabilized glass regains most of its original light
transmittance during cell downtime. Leaded, but unstabilized glasses, are also
self-restoring to a lesser degree.
A browning problem in a window tank can be alleviated only by
replacing the glass slabs with others more resistant to the browning
effect.
Anti-Reflective Surfaces
Leaded glass that has been chemically etched
to reduce reflection presents a particular cleaning problem, and
should be performed by the window glass manufacturer. Improper
cleaning operations may result in an "orange-peel" effect
on the glass surface, which causes distortion and very poor viewing
capabilities. The only cure for orange peeled glass is to
disassemble the window and repolish the slab on a polishing machine.
Oil Leakage
Leakage of the window tank oil on 1950-1970 vintage
windows may be caused by either a cracked weldment in the tank
structure or a leaking gasket under the tank cover plates.
A cracked weldment on older window housings can be repaired only
by removing the tank unit and determining the location and extent of
the crack. The crack may be repaired with a glass epoxy compatible
with the mineral oil, or by rewelding. Rewelding may require special
precautions and procedures. Under the NQA-1 Quality Assurance program at Premier
Technology, Inc. a cracked weldment is not a problem to be
encountered on a new window housing.
If the leakage is due to a gasket leak, it is generally corrected
by retorquing the compression bolts and increasing the compressive
force on the gasket. Many gasket materials slowly deform over
time, causing a relaxation of the compressive sealing force. If the
leak persists, the gasket must be replaced, and possibly the
clamping frame also.
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Data and information contained in the tutorial was written by Dale A. Tobias, Premier Technology, Inc. and Hienz E. Hoffman and William G. Wash, Schott Glass Technologies, Inc. and may be reproduced only with written consent.
For information, contact Lyle Freeman
Vice President of Business Development
(208) 782-9129 lfreeman@ptius.net
