The problem, say materials scientists at Lehigh University, has nothing to do with your dishwasher or your standards of cleanliness. And while the type of detergent you use may aggravate the dirty appearance, forces of fundamental science are more to blame.
Researchers at Lehigh University have discovered that the milky band is actually a network of microscopic scratches that form as the glass is being manufactured. These cracks are not visible when you examine your new wineglass against the light before purchasing it.
But repeated washings in the dishwasher cause the glass in the wineglass "bowl" to dissolve slightly. This in turn causes the tiny cracks to spread and to scatter light, giving the bowl what looks like a recalcitrant swath of dirt or grime.
The researchers published their findings in the October issue of the Journal of the American Ceramics Society in an article titled "Influence of the Manufacturing Process on Corrosion Behavior of Soda-Lime-Silicate Glassware."
The three-year study was funded by Unilever, an international maker of household consumer products. It was led by Himanshu Jain, Diamond professor and chair of materials science and engineering at Lehigh. Jain's co-authors on the JACS paper were Anju Sharma, a Lehigh graduate student, and Joseph Carnali and Guillermo Lugo of Unilever Research U.S. in Edgewater, N.J.
Jain began the study by assembling a variety of new, not-yet-washed wineglasses. "The as-received glassware appeared clear and transparent to the unaided eye," he wrote in the JACS article. Upon closer examination with optical microscopy however, he noticed "grooves and scratches of sub-micrometer size along the circumference of the bowl."
"The existence of these microscopic surface defects, even before washing, suggests that they were created during the manufacturing process and/or subsequently to forming during handling," the researchers wrote.
After making this discovery, Jain and his colleagues washed the glasses for as many as 100 cycles in three detergent solutions ranging from benign to harsh. They observed that the glasses washed in the benign solution containing no sodium disilicate remained clear and transparent.
The glasses washed in solutions containing .7 grams and 1.5 grams of sodium disilicate per liter, however, became visibly corroded around the center of the glass bowl - "exactly where the scratches and grooves were found before the sample was washed," the researchers noted. Glasses washed with the harsher solutions also became tinged with blue and other colors near the rim, they said.
Ironically, Jain's group found that the most aggressive dishwashing solutions do not cause the most visible scratching. That is because these solutions cause all of the glass to dissolve at the same fast rate. The resulting bowl is thinner but still transparent, and may break in the dishwasher before it takes on the telltale cloudy look of corrosion.
Most consumers who file complaints, says Jain, are using a mid-range dishwashing detergent, which causes the pre-formed scratches on the glass to dissolve more quickly than the rest of the glass, and thus acquire the corroded, or cloudy look.
Wineglass and dishwashing detergent companies receive more complaints in Europe than in the U.S., says Jain. This may be due to the fact that, especially in Germany and The Netherlands, dishes are washed at higher temperatures than in the U.S., which enhance the problem.
Carnali said Unilever has made "subtle" changes in the composition of its dishwashing detergents as a result of the study in order to reduce the amount of the ingredients known to cause corrosion. Carnali also said that washing the wineglasses by hand prevents the glass bowl from clouding.
In summary, says Jain, the problem of corroded wineglasses is two-pronged. It can be mitigated either by controlling the manufacturing process, especially as the glass makes the transition from liquid to solid, or through the use of improved detergent formulations.
"A detergent manufacturer has taken the first step," said Jain. "Now it is the glass manufacturers' turn to address this quality control issue."