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Cooling Marks - Appearances can be deceiving

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manddmoir's loves254 of 954Loetz Metallrot (Metallic Red), Prod. Nr. I/7569, ca. 1898Loetz Oceanik Vase
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    Posted 11 years ago

    bohemiangl…
    (640 items)

    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.

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    Comments

    1. LOUMANAL LOUMANAL, 11 years ago
      I would probably shy away from the line/crack on the 3rd photo but the others I would probably consider as acceptable especially on a rare/older piece. BOB
    2. bohemianglassandmore bohemianglassandmore, 11 years ago
      Thanks - glad your Loetz worked out, Gary!

      Bob, there is the cosmetic problem - The piece in the third photo is a beautifully enameled signed Moser, and the line is not even a solid line - it's just a wispy, feathery inclusion that is definitely not a crack. It's a seller's nightmare, at least on eBay. It think with good photos and an honest explanation, it could be overcome.

      Of course we have to be honest with ourselves - sometimes a crack is a crack!
    3. Moonstonelover21 Moonstonelover21, 11 years ago
      Warren, great share and imparting of knowledge...thank you!
    4. smiata smiata, 11 years ago
      Thanks for the lesson, and checked marked under new information. Always learning.
    5. AmberRose AmberRose, 11 years ago
      Great info, thanks!

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