Black Light Testing

By Staff Researchers at AntiqueResources.com

Black light testing is a common practice used to authenticate antiques, to determine the extent of repairs and to look at cool posters on black velvet.

For many beginning collectors, the science and techniques used in the black light testing method are a mystery, this article hopes to address this issue.

Blacklights are used in evaluating antiques because the ultraviolet rays they produce react differently to different materials. Because of this interesting characteristic, things that are invisible to the naked eye become visible under the blacklight.

Before we go any further, a word of warning: Be sure that the blacklight you purchase is a longwave blacklight. Shortwave blacklights can damage your eyes and skin.

Now, onto the uses of blacklights in collecting antiques. Different chemical properties become apparent under a blacklight, modern paint will flouresce under a black light, older paints will not, you can use this to determine whether a painted object is an antique or a newer reproduction as well as to determine whether a piece has been "touched up" and if so, how extensive was the repair.

The same technique can be used to detect repairs on antique porcelain as the old finish will not glow under a blacklight, and the newer material in the repair will.

Some antique glassware will glow under the blacklight as well, vaseline glass will glow because it contains uranium oxide.

A blacklight can be used to test many types of antiques. Here are some examples:

Cut Glass: Authentic American Brilliant period cut glass fluoresces yellow; reproductions have no reaction or show a blue-white.
Editor's Note: This appears to be in debate, we recently received an email from a member of the American Cut Glass Association who informs us that "American Brillant cut glass does not show yellow with long wave black light - it shows either pale violet or pale blue. More modern shows bright blue, almost electric. We have over 300 pieces and only one shows yellow and it is a European (English) piece. The use of black light with cutglass is HIGHLY controversial even among ACGA folks. Some swear by it, others say it is unreliable. I'm with the latter school as it is well known that many US cutglass houses had some of their blanks made in England and some even in Bohemia - this of course shoots the only American cut glass flouresces a certain color theory all to the devil!!"

Cast Iron: Most new paint on most new cast iron fluoresces; old paint on old cast iron rarely fluoresces. You can also detect newly painted repairs as well.

Paper Products: Most paper products--like post cards, books, signs, photos papers, etc.--made before the late 1930s-WW2 era rarely fluoresces. Paper products made since 1950, however generally fluoresces brightly due to large amounts of chemical bleaches and dyes.

Pattern Glass: Nearly all American colorless pressed glass made before ca. 1925-30 fluoresces yellow; reproductions generally do not have any fluorescence.

Textiles: Thread and Cloth: Synthetic fibers--rayon, polyester, etc. made since WW2 all fluoresce. This allows you to detect new military uniforms, new quilts, new doll clothes, etc. Even if old fabric is used to repair an old quilt, the new thread will fluoresce.
Editor's Note: We received an email from Alan Kelchner, a professional quilt restoration specialist who added the following information: "A quilt from 1800 may actually fluoresce, while a quilt made in November 1998 may not. Mass produced laundry detergents from the current and past decades have contained additives that make your laundry whiter and brighter. These additives can make textiles fluoresce under a black light, which would lead a person to believe an antique quilt/textile, washed in these detergents, was newly made. And while many newly made fabrics and threads do fluoresce, there are also fabrics and threads whose manufacture do not cause it to fluoresce.

Man-made fibers available in the Depression era can be found in all sorts of vintage quilts from that era. They are very collectible, and the use of fiber content to label them new is poor information for the novice collector.

A collector must learn their textile field and be knowledgeable. Blacklighting is definitely a poor way to confirm a textile's age."

Art Glass: Lalique before 1945 fluoresces yellow; after 1945, it does not. Genuine old Burmese fluoresces bright yellow green, new reproductions do not.

Porcelain: Hard paste porcelain will flouresce a deep blue or purple color. Soft paste will flouresce white.

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