streaks
Cords (also known as reams, striae, streaks or colour streaks) are glass defects caused by a lack of homogeneity in the glass melt. They may result from variations in the raw materials used, irregularities during melting, the high viscosity of the melt or temperature differences within the melting pot.
The
goblet (maker unknown, 1600–1700) in the Museum Boijmans Van Beuningen collection provides an example of a cord in the form of a distinct colourless spiral in the red glass.
This occurred because the glass melt was not homogeneous or because the glass composition caused ingredients to evaporate from the surface of the melt, changing the viscosity of the thin top layer of the glass. When glass is gathered from the surface and subsurface of the melting pot, spiral cords can form as the glass is shaped while rotating the blowpipe.
Certain patterns that become visible under special lighting closely resemble cracks. Distinguishing between cords and cracks can be particularly difficult in glass where the cords and cracks run in the same direction. See
vulnerabilities - atmospheric degradation - misleading characteristics
