Collectors may specialize in one of several types of paperweights, but more often they wind up with an eclectic mix.
Millefiori (Italian - "thousand flowers") paperweights contain thin cross-sections of cylindrical composite canes made from colored rods and usually resemble little flowers, although they can be designed after anything, even letters and dates. These are usually made in a factory setting. They exist in many variations such as scattered, patterned, close concentric or carpet ground. Sometimes the canes are formed into a sort of upright tuft shaped like a mushroom that is encased in the dome. The year of manufacture is sometimes enclosed in one of the canes.
Lampwork paperweights have objects such as flowers, fruit, butterflies or animals constructed by shaping and working bits of colored glass with a gas burner or torch and assembling them into attractive compositions, which are then incorporated into the dome. This is a form particularly favored by studio artists. The objects are often stylized, but may be highly realistic.
Sulfide paperweights have an encased cameo-like medallion or portrait plaque made from a special ceramic that is able to reproduce very fine detail. These are known as incrustations, cameo incrustations, or sulphides. They often are produced to commemorate some person or event. From the late 1700s through the end of the 1900s, an amazing variety of glass objects, including paperweights, were made with incrustations. The finest collection of incrustations ever assembled was by Paul Jokelson, collector, author and founder of the Paperweight Collectors' Association. A part of his collection was gifted to the Corning Museum of Glass, with the remaining portion being sold in London in the 1990s. Although still produced today, their heyday was before the classic period.
Most paperweights, which are considered works of art, use one of the above techniques; millefiori, lampwork or sulphide — all techniques that had been around long before the advent of paperweights. A fourth technique, a crimp flower, usually a rose, originated in the Millville, New Jersey area in the first decade of the twentieth century. Often called a Millville rose, these weights range from simple folk art to fine works of art, depending on the maker.
Fine weights not made with any of the major techniques include swirls, marbries and crowns. Swirl paperweights have opaque rods of two or three colors radiating like a pinwheel from a central millefiori floret. A similar style, the marbrie, is a paperweight that has several bands of color close to the surface that descend from the apex in a looping pattern to the bottom of the weight. Crown paperweights have twisted ribbons, alternately colored and white filigree which radiate from a central millefiori floret at the top, down to converge again at the base. This was first devised in the Saint Louis factory and remains popular today.
Antique Clichy Green & White Swirl Paperweight With Large Millefiori Center. Made in France in the Mid 1800s.
Miniature weights have a diameter of less than two inches or so, and magnums have a diameter greater than about 3.25 inches.
California-style paperweights are made by "painting" the surface of the dome with colored molten glass (torchwork), and manipulated with picks or other tools. They may also be sprayed while hot with various metallic salts to achieve an iridescent look.
Victorian portrait and advertising paperweights were dome glass paperweights first made in Pittsburgh, Pennsylvania using a process patented in 1882 by William H. Maxwell. The portrait paperweights contained pictures of ordinary people reproduced on a milk glass disk and encased within clear glass. This same process was also used to produce paperweights with the owner's name encased or an advertisement of a business or product. Pittsburgher Albert A. Graeser, patented a different process for making advertising paperweights in 1892. The Graeser process involved sealing an image to the underside of a rectangular glass blank using a milk glass or enamel-like glaze. Many paperweights of the late 19th century are marked either J. N. Abrams or Barnes and Abrams and may list either the 1882 Maxwell or 1892 Graeser patent date. It has been theorized that Barnes and Abrams did not actually manufacture advertising paperweights for their customers, but instead subcontracted the actual manufacturing task out to Pittsburgh area glasshouses. The Paperweight Collectors Association Annual Bulletins published in 2000, 2001 and 2002 describe these in detail.
Bohemian paperweights were particularly popular in Victorian times. Large engraved or cut hollow spheres of ruby glass were a common form. see wikipedia