Glass Coloring Techniques*

There are numerous ways to add color in the manufacture of glass. The most common method is to mix different oxides into the batch to give it a uniform color. This is done to produce glass such as milk glass, cranberry, cobalt, amber, opaline, etc. There are basically three different types of colored glass: transparent, translucent and opaque. Transparent glass allows light to pass through without diffusion. Objects can be seen through it clearly. Translucent glass allows light to pass through in a diffused manner. Since only limited light passes through, objects appear unclear. Opaline glass falls into this category. Opaque glass is solid-colored glass that allows little or no light to pass through.

Staining

Another glass-coloring method is called staining. This is accomplished after a metallic oxide is applied to the body and the glass is reheated. The oxide provides the color and leaves a thin layer on the surface of the glass. By scratching, cutting, or engraving this layer, the clear glass underneath is revealed. This can create many different effects commonly called cut-to-clear. The colors created are quite bold and give the same appearance as solid-colored glass or overlays.

Stained glass steins, mid 1800s.

Lythalin glass (opaque), c. 1840; lustre vase, Loetz, c. 1910.

Overlay-Flashed

Another coloring method is the overlay process. This is achieved when the glass blower has a ball of glass on the end of his pipe and inserts that ball into a pot of different-colored glass inside the furnace. This second color will then encase the first layer. This can be done several times to achieve numerous layers. This overlay process can be done with freehand-blown or mold-blown pieces, but is usually done with mold-blown.

Double overlay, blue on white on clear; single overlay, white on green, c. 1870.

Clear glass stein, c. 1900; cobalt blue stein c. 1840; white opaline stein, mid 1800s.

Lustre

A lustre is achieved by tinting the exterior of the glass with a metallic oxide vapor. A small amount of the oxide is inserted into the kiln in a special container or a thin layer is painted or sprayed onto the body. The heat in the kiln transforms the oxide into a gas which is trapped in the kiln and coats the clear glass surface with a thin layer of iridescent color.

Cased

Top rim of cased glass stein showing the two equal layers of glass.

Cased glass is a less common process utilizing two or more layers of glass of different colors. This process involves first making the outer casing by blowing a gather, knocking off one end, and opening the piece to form a cup-like shell. This shell is then placed into a metal mold and a second gather, of a different color, is blown into it. This combined piece is taken from the mold and reheated so that the two layers fuse together. The outer layer is thick, as distinguished from the thin outer layer of flashed glass. This term is also improperly used in place of flashed or overlay to imply one layer of glass over another.

Enamel Overlay

Left - handle attachment onenamel overlay stein; Right - handle attachment on glass overlay stein.

Enamel overlay is a process whereby the entire glass body is enameled to give the appearance of a glass over lay. This enameling can be cut through to reveal the layer underneath. The enamel is opaque. It is not applied until the glass item is completed. When examining a stein, one need only look at where the handle is attached to the body. Since the enamel is applied after the handle is attached, there is a void of color between the body and the handle. With a true glass overlay, the glass layer on the body is added first, before attaching the handle. Therefore, the colored glass layer will be found between the handle and the body, as well as covering the entire body. To understand this difference more clearly, examine the examples to the left.

__________

*Reprinted from The Beer Stein Journal, February 1995, by permission from Gary Kirsner Auctions.