Platinum Metals Rev., 1973, 17, (1), 25
The Surface Tension of Molten Glass
A Modified Dipping Cylinder Method
The detachment method of measuring surface tension was first employed by Wilhelmy in 1863 and was later developed by du Noüy into a commercial instrument. This typically consists of a horizontal iridium-platinum ring attached to one arm of a balance. The ring is then immersed in the liquid. The force required to pull the ring from the surface of the liquid is then measured and is proportional to surface tension.
Du Noüy’s method has been applied successfully to the measurement of the surface tension of glass, with the important difference that the ring has evolved into a shallow cylinder. Hitherto some form of balance has been retained with attendant difficulty of usage. This objection has now been overcome by R. L. Tiede of Owens Corning Fiberglas Corp., who measures the pull on the cylinder with a load cell (Am. Ceram. Soc. Bull., 1972, 51, (6), 539–541). Not only is the new apparatus much quicker and easier to use but it is capable of giving results reproducible to within ± 1 per cent.
The glass, usually about 300 g, is melted in an electrically heated platinum alloy container. The cylinder is attached by a thin wire to the load cell which is slowly raised and lowered by a reversible electric motor. Initially the cylinder is suspended just above the surface of the glass, and a chart recorder, to which the load cell is connected, is adjusted for zero and full scale deflection as appropriate to the type of glass being studied. The cylinder is now lowered until its entire circumference is in contact with the glass; it is then withdrawn from the melt, allowing a characteristic trace to be plotted up to the point of separation from the surface.
The maximum value of the downward force experienced by the cylinder is obtained by calculation from this trace. The surface tension is then obtained by substituting this value into an equation relating surface tension to the dimensions of the cylinder, the downward force and the density of the glass.
The apparatus is sufficiently versatile to permit the rapid determination of surface tension at different temperatures and also to measure viscosity and electrical conductivity before the glass is finally drained from the furnace.