Milk glass–an opaque glass aptly named for its milky-white color–dates back as early as the 1500s, but American manufacturers caught on to the trend in the 1900s by producing glass tableware as a less-expensive substitute for porcelain. The Depression Era “look for less” quickly gained popularity and spread to more affluent homes. By the time the Gilded Age rolled around, glass manufactures started creating more intricate pieces and patterns retailing for higher prices.

Today, the once affordable knock offs can fetch upwards of $200 per item, and an extremely rare piece can go for several thousand dollars. But beware newbie collectors. Remember those tacky white vases with raised bumps from the local florist? While technically considered milk glass, these commonplace vases will earn you about 50-cents at a garage sale.

In fact, milk glass is a fairly recent term that encompasses many forms, qualities and colors of opaque glass manufactured over the years. Early glass makers used arsenic to create a faintly grayish but nicely opalescent look. Later manufacturers created a denser, whiter glass using tin oxide, feldspar and other additives. The change to the glass recipe makes it possible to determine the age of a piece by examining its edges. Older milk glass will appear nearly transparent at the edge while newer pieces are bright white and may not allow light through.

The various colors of milk glass–blue, pink, yellow, brown and black¬–can also indicate the age and value of a piece. For example, a pink platter may be a piece from the widely collectible American Sweetheart pattern manufactured by the Macbeth-Evans Glass Company in the 1930s.

But it may not.

Bottom line: unless you have a signed McKee animal-covered dish on your hands–a purebred of all milk glass–it’s not always easy to determine if milk glass is rare. To make matters worse, many American manufacturers did not sign their early pieces.

There are a number of resources available intended to demystify the murky waters of milk glass collecting, including the popular Collector’s Encyclopedia of Milk Glass by Betty and Bill Newbound (Collector Books, 1994). But even avid collectors recommend leaving the guesswork to a reputable appraiser.

If the prospect of beginning a collection is too daunting, consider accenting your home with modern reproductions or newly manufactured pieces. Many popular home decor chains sell milk glass in the form of lamps and canisters. You can also purchase a quality original directly from the source; the Fenton Art Glass Company of Williamstown, West Virginia is one of the few glass makers that continue producing high-quality milk glass.