Cooling Marks - Appearances can be deceiving

Usually, when a piece of glass is cracked, there is an apparent reason for it - either a nearby chip, impact mark, or stress where a handle is applied. Sometimes, a line will appear in a piece where there is no apparent reason for it, in a place that doesn't make sense. In those cases, what you may have is a cooling mark, and if so, it isn't a crack, nor is it damage in any way. It is a cosmetic blemish created when glass surfaces cool at different temperatures. This can be the result of the hot glass coming into contact with a tool that is much cooler. Even though the glass continues to be molded, blown, stretched - the area where it touched the tool will refract light differently, and as it is expanded into shape, it can appear as a shadowy line inside the glass that, at first glance, appears to be a crack.

The photos above are all cooling marks on pieces in my collection - I have looked at several of these types of marks under strong lighting and with jeweler's loupes, and have noticed a few things. First of all, most cooling marks cannot be felt, as they are almost always beneath the outer surface of the glass. Some are more obvious than others. When they are on the surface, they will be felt as a slightly raised surface, not like a crack, which is a structural gap in the glass. Secondly, upon close scrutiny, cooling marks may have gaps or appear feathery or cloudy, or even swirl and fade into the glass body, which is definitely not how a crack would look.

One online source, glassblowers.org, describes it best: Any time an instrument is used on hot glass, there's the potential for it to leave a cooled surface that refracts light differently.

In summation, cooling marks are a good news/bad news story... the good news is that something that may seem to be a crack is actually no damage at all, but cosmetically, it still may pose a problem for sellers.