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12 November 2003

Microscopic scratches can lead to cloudy glasses

Wineglasses that come out of the dishwasher cloudy may not necessarily be the result of lousy detergent or a weak dishwasher, but instead the blame may lie with the manufacturing process.

Although the type of detergent you use may aggravate the dirty appearance, materials scientists at Lehigh University said forces of fundamental science are more to blame.

Researchers at Lehigh discovered that the milky band found in the glass bowl is actually a network of microscopic scratches that form as manufacturers make the glass. 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 a dirty appearance.

Unilever, an international maker of household consumer products, funded the three-year study led by Himanshu Jain, Diamond professor and chair of materials science and engineering at Lehigh.

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 said. 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," Jain's team wrote in a paper.

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 0.7 grams and 1.5 grams of sodium disilicate per liter, however, 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 turned blue and other colors near the rim, they said.

Ironically, Jain's group found 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, Jain said, are using a midrange dishwashing detergent, which causes the preformed 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., said 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.

Unilever's Joseph Carnali said the company made "subtle" changes in the composition of its dishwashing detergents as a result of the study to reduce the amount of the ingredients known to cause corrosion. Carnali also said washing the wineglasses by hand prevents the glass bowl from clouding.

For more information on this subject go to www.isa.org/measurement.