Furniture Detective: How to tell the age of furniture glass

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A question that invariably pops up during the examination of an older or antique piece of furniture is “Is that the original glass?”

It’s relatively easy to determine if a piece has been refinished — it lacks the normal wear and tear evident on an old finish. And it’s 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? It doesn’t wear with age, doesn’t smell when its new, and you can’t repair it, so how can you tell?

There are two inspection techniques to determine if a piece of glass or a mirror has been replaced. The first technique looks at the support structure around the glass; 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 that 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 the exposed edge of the muntin is covered in putty or glazing material. Look for signs of new putty around the muntins and tool 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 won’t 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 that 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, professionals and amateurs 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, which are flat, diamond-shaped pieces of metal driven into the frame to 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. Is the color clear, or does it have a greenish tint? Check the number of seeds or imperfections and the clarity of individual panes of glass.

Glass-making techniques from different periods leave distinctive patterns of distortion in the glass. If you know the patterns, you can tell the age 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 basically flat. Crown glass has a circular swirl pattern in it from the spinning motion.

But in the 19th century, the technique was changed to 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 a flat sheet. (See the cylinder glass making video at the bottom of the page.)

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 parallel wavy lines of distortion in old clear glass. Perfectly flat glass (within 1/25,000 inch) was perfected in the late 1950s by pouring molten glass on a still bed of molten tin. This is called “fire-polished” glass and is the most prevalent today. ?

Send your comments, questions and pictures to Fred Taylor, P.O. Box 215, Crystal River, FL 34423 or Visit Fred’s Web site:

His book “How To Be a Furniture Detective” is available for $18.95 plus $3 S&H. Also available is Fred and Gail Taylor’s DVD, “Identification of Older & Antique Furniture,” ($17 + $3 S&H) and a bound compilation of the first 60 columns of “Common Sense Antiques by Fred Taylor” ($25 + $3 S&H). For more information call 800-387-6377, fax 352-563-2916 or


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