A question that invariably pops up during the examination of an older or antique piece of furniture is "Is that the original glass?". Its relatively easy to determine if a piece has been refinished - it lacks the normal wear and tear evident on an old finish. And its also easy to determine if a piece has been re upholstered. New fabric, new stuffing and new gimp are undeniable in appearance and smell. Even well done wood repairs can be detected with an exacting enough inspection but how can you tell about glass? After all, glass is glass isn't it and it doesn't wear with age, doesn't smell when its new and you can't repair it so how can you tell?
Actually there are two basic inspection techniques you can use to determine if a piece of glass, or a mirror for that matter, has been replaced. The first technique looks at the support structure around the glass and the second looks at the glass itself.
Glass incorporated into a piece of furniture has to be supported and attached in some manner. The most common method of securing clear window glass in a cabinet is with wooden strips nailed into the case or door frame that hold the glass in place. In older furniture these wooden strips are very often brittle and can tell you if they may have been removed to replace the glass. Carefully inspect the strips for signs of removal which might include indentations left by the screwdriver or knife used to pry the strips from their original home. Also look for jagged breaks in longer strips that may indicate some rough use somewhere along the line. Lastly look for a second set of nail holes in the strips. Very often a good repair person will put the original nails back in the original holes but sometimes that can't be done for a variety of reasons and new nails in new holes have to be installed. These will be obvious with a close, critical look.
Another, older method of securing glass in place is with the use of original muntins, not to be confused with the plywood cutouts of the same name used in 20th century reproductions. Original muntins are strips of wood which surround a piece of glass in the center of a door and hold several individual pieces of glass in place to make a glass paneled door. Usually the glass is held to the muntins with a tiny headless nail and then the exposed edge of the muntin is covered in putty or glazing material. Look for signs of new putty around the muntins and tools marks in the surrounding areas, indicating the nails may have been removed and replaced.
Checking for original mirrors is usually even easier than looking for new clear glass. Most mirrors have a backing material of some sort over the frame in the rear. It may be just paper glued over the frame or it may be wood. If the old paper has been replaced you know right away that there is a good chance the old mirror has been replaced also. If the back panel is wood make sure it is consistent with the purported age of the frame. For example, a mid 19th century mirror will not originally have had a plywood back panel. It should have a solid board or several boards nailed in place to make the panel. Again the nails are important. Mid 19th century nails are different from mid 20th century nails so new nails are a big clue. So are the nail holes. A second set of holes or holes of the wrong shape or size could mean the panel has been removed and the mirror replaced or re silvered.
Then take a look at how the mirror itself is held in place in the frame. Most older mirrors are held in by triangular shaped blocks which were glued in place with the point of the triangle facing in toward the frame and the base of the triangle sticking out toward the back panel. Look for evidence of the blocks having been moved and re glued or re nailed. Many repair people, professional and amateur alike, will not even fool with the old triangular glue blocks. They just cut square blocks and nail them in or use modern metal glazier's points, flat, diamond shaped pieces of metal driven into the frame so that it hold the glass in tightly. Flat metal glazier's points are 20th century technology so they cannot be original to a 19th century mirror.
Finally, examine the glass itself. Make sure it is consistent with other glass in the piece if there is any and see if it matches the glass in other pieces from the same period. Keys points to compare are the color, is it clear or does it have a greenish tint, the number of seeds or imperfections and the clarity of individual panes of glass. Different glass making techniques from different periods leave their own distinctive patterns of distortion in the glass and if you know the patterns you can tell the age, more or less, of the glass.
Glass made prior to the 19th century was called crown glass, made by spinning a disk of molten glass until it was more or less flat. Crown glass has a circular swirl pattern in it from the spinning motion. 19th century was mostly cylinder glass made by swinging a blown bubble of molten glass rather than spinning it. Swinging the glass produced a cylinder which was cooled, scored down one side and reheated. As it reheated it laid itself out in more or less flat sheet. Cylinder glass has an evenly distributed mottled, dimply distortion pattern. Early 20th century glass was pulled from the kiln and passed between iron rollers to flatten it. That produced the parallel wavy lines of distortion in old clear glass. Perfectly flat glass (within 1/25,000 of an inch) was perfected in the late 1950's by pouring molten glass on a still bed of molten tin. This is called "fire polished" glass and is the most prevalent today.
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