Glass fusing methods involve stacking, or layering thin sheets of glass, often using different colors to create patterns or simple images. This will take place at temperatures between 1100° and 1700° Fahrenheit (600° to 925° Celsius). Fusing is the process of using a kiln to join together pieces of glass. When continuation of heat applied to the glass, the glass will become more fluid and flow together. The longer the kiln is held at the maximum temperature the more thoroughly the stack will fuse, eventually softening and rounding the edges of the original shape.

Glass Etching

In the 1920s a new mould-etch process was invented, in which art was etched directly into the mould, so that each cast piece emerged from the mould with the image already on the surface of the glass. This reduced manufacturing costs and, combined with a wider use of colored glass, led to cheap glassware in the 1930s, which later became known as Depression glass. As the types of acids used in this process are extremely hazardous, abrasive methods have gained popularity.

Typically, “glass etching cream” available from art supply stores consists of fluoride compounds, such as sodium fluoride and hydrogen fluoride (very dangerous). The fluoridation of the glass causes the characteristic rough, opaque qualities of frosted glass.

Abrasive sandblasting is also a common technique for creating patterns in glassware, creating a “frosted” look to the glass. High pressure air mixed with an abrasive material cuts away at the glass surface to create the effect. The longer the stream of air and abrasive material are focused in one spot, the deeper the cut will be. Sandblasting is often used for removing unwanted items that have stuck to the glass or as a decorative purpose on gifted and personal items.

Stained glass has had various levels of popularity throughout history. The 12th and 13th centuries in Europe have been designated as the Golden Age of Stained Glass. However, during the Renaissance period, stained glass was replaced with painted glass, and by the 18th century it was rarely, if ever, used or made according to medieval methods. During the second half of the 19th century, European artists rediscovered how to design and work glass according to medieval principles, and large quantities of stained glass windows were made.

In America, the stained glass movement began with William Jay Bolton, who made his first window for a church in New York in 1843. But he was to be in the business for only six or seven years before returning to his native England. No other American practiced the art professionally until Louis Comfort Tiffany and John La Farge began working with stained glass near the end of the 19th century. In fact, the art of stained glass in the United States languished until the 1870s, and did not undergo a true revival until the turn of the century. At this time, American architects and glassmen journeyed to Europe to study medieval glass windows, returning to create similar art forms and new designs in their own studios.

A leaded stained glass window or other object is made of pieces of glass, held together by lead. The pieces of glass are about 1/8-inch (3.2 mm) thick and bound together by strips, called “cames” of grooved lead, soldered at the joints. The entire window is secured in the opening at regular intervals by metal saddle bars tied with wire and soldered to the leads and reinforced at greater intervals by tee-bars fitted into the masonry. A faceted glass panel differs slightly from traditional leaded stained glass in that it is made up of pieces of slab (dalle) glass approximately 8 inches square, or in large rectangular sizes, varying in thickness from 1-2 inches (2.5-5 cm). These slabs are not held together with lead; rather they are embedded in a matrix of concrete, epoxy, or plastic.

Raw Materials

Glass is made by fusing together some form of silica such as sand, an alkali such as potash or soda, and lime or lead oxide. The color is produced by adding a metallic oxide to the raw materials.

Copper oxide, under different conditions, produces ruby, blue, or green colors in glass. Cobalt is usually used to produce most shades of blues. Green shades can also be obtained from the addition of chromium and iron oxide. Golden glass is sometimes colored with uranium, cadmium sulfide, or titanium, and there are fine selenium yellows as well as vermilions. Ruby colored glass is made by adding gold.

The Manufacturing Process

Stained glass is still made the same way it was back in the Middle Ages and comes in various forms. For the glass used in leaded glass windows, a lump of the molten glass is caught up at one end of a blow pipe, blown into a cylinder, cut, flattened and cooled. Artisans also vary this basic process in order to produce different effects. For example, “flashed glass” is made by dipping a ball of molten white glass into molten colored glass which, when blown and flattened, results in a less intense color because it will be white on one side and colored on the other.

So-called “Norman slabs” are made by blowing the molten glass into a mold in the shape of a four-sided bottle. The sides are cut apart and form slabs, thin at the edges and as much as 0.25 inch (0.6 cm) thick) at the center. Another form of glass, known as cathedral glass, is rolled into flat sheets. This results in a somewhat monotonous regularity of texture and thickness. Other similarly made glasses are referred to as marine antique, but have a more bubbly texture.

Processing the stained glass

Large manufacturers of stained glass mix the batch of raw materials, including alkaline fluxes and stabilizing agents, in huge mixers. The mix is then melted in a modern furnace at 2,500° F (1,371° C). Each ingredient must be carefully measured and weighed according to a calculated formula, in order to produce the appropriate color. For cathedral glass, the molten glass is ladled into a machine that rolls the glass into 1/8-inch (3.2 mm) thick sheets. The sheets are then cooled in a special furnace called an annealing lehr. The glass is then inspected, trimmed to standard size, and packed into cases.

At a typical factory, eight to ten different color runs are made per day. Some manufacturers cut a small rectangle of glass from each run in order to provide a sample of each color to their customers. There are hundreds of colors, tints, and patterns available, as well as a number of different textures of cathedral glass. Different textures are produced by changing the roller to one having the desired texture. Glass manufacturers are continuously introducing new colors and types of glass to meet the demands of their customers.

Creating the window pattern

Though some of the tools to make stained glass windows have been improved, the windows are still hand crafted as they were centuries ago. The first step of the process involves the artist creating a small scale version of the final design. After the design has been approved, the craftsperson takes measurements or templates of the actual window openings to create a pattern. This pattern is usually drawn on paper or cardboard and is the actual size of the spaces to be filled with glass.

Next a full-sized drawing called the cartoon is prepared in black and white. From the cartoon, the cutline and pattern drawings are made. The modern cutline drawing is a careful, exact tracing of the leadlines of the cartoon on heavy paper. The leadlines are the outlines of the shapes for patterns to which the glass is to be cut. This drawing serves as the guide for the subsequent placing and binding with lead of the many pieces of glass.

The pattern-drawing is a carbon copy of the cutline drawing. It is cut along the black or lead lines with double-bladed scissors or a knife which, as it passes through the middle of the black lines, simultaneously cuts away a narrow strip of paper, thus allowing sufficient space between the segment of glass for the core of the grooved lead. This core is the supporting wall between the upper and lower flanges of the lead.

Cutting and painting

Colored glass is then selected from the supply on hand. The pattern is placed on a piece of the desired color, and with a diamond or steel wheel, the glass is cut to the shape of the pattern. After the glass has been cut, the main outlines of the cartoon are painted on each piece of glass with special paint, called “vitrifiable” paint. This becomes glassy when heated. The painter might apply further paint to the glass in order to control the light and bring all the colors into closer harmony. During this painting process, the glass is held up to the light to simulate the same conditions in which the window will be seen. The painted pieces are fired in the kiln at least once to fuse the paint and glass.

Glazing and leading

The next step is glazing. The cutline drawing is spread out on a table and narrow strips of wood called laths are nailed down along two edges of the drawing to form a right angle. Long strips of grooved lead are placed along the inside of the laths. The piece of glass belonging in the angle is fitted into the grooves. A strip of narrow lead is fitted around the exposed edge or edges and the next required segment slipped into the groove on the other side of the narrow lead. This is continued until each piece has been inserted into the leads in its proper place according to the outline drawing beneath.

Finishing

The many joints formed by the leading are then soldered on both sides and the entire window is waterproofed. After the completed window has been thoroughly inspected in the light, the sections are packed and shipped to their destination where they are installed and secured with reinforcing bars.

Faceted glass

For faceted glass windows, the process begins the same way, with the cutline and pattern drawings being made with carbons in a similar manner. The pattern drawing is then cut to the actual size of the piece of glass with ordinary scissors since there is no core of lead to allow for. The thick glass slabs next are cut with a sharp double-edged hammer to the shape of the pattern. To give the slab an interesting texture, the worker then chips round depressions in the glass with the same hammer. This is called faceting.

Instead of glazing with lead, a matrix of concrete or epoxy is poured around the pieces of glass. The glass pieces have first been glued to the outline drawing, which is covered with a heavy coating of transparent grease so that the paper can be removed after the epoxy sets. The whole is enclosed within a wooden form, which is the exact size and shape of the section being made. The worker must wear gloves during this process, since epoxy resin is a toxic material. After hardening, the section is cleaned and cured prior to shipping and installation.

The process for making an entire stained glass window can take anywhere from seven to ten weeks, since everything must be done by hand. Cost can vary widely depending on complexity and size, though some windows can be created for a cost as low as $500. The customer can choose an existing pattern rather than create an entirely new one to minimize costs. In this case, the pattern can be customized by altering shapes or by changing the placement of the central image.

Another term for these processes is kiln-forming. It’s not unusual for the two terms—“warm glass” and “kiln-forming”—to be used interchangeably.

Fusing and Slumping

Glass fusing is the process of using a kiln to join together pieces of glass. If you apply heat to glass, it will soften. If you continue to apply heat, the glass will become more fluid and flow together. Two or more pieces of glass will stick (or “fuse”) to each other. When the right kind of glass is heated and then cooled properly, the resulting fused glass piece will be solid and unbroken.

Many people also use the word “fusing” to include bending and shaping glass using the heat of a kiln. This manipulation can take many forms, but the most common is slumping, where a mold is used to cause already fused glass to take on the shape of a bowl, a plate, or similar object. Other kinds of manipulation done with fusing techniques are combing, which involves using a tool to distort the shape of the glass while it is hot, and fire polishing, which uses a kiln to heat the glass just enough to make it shiny and smooth.

Another category of kiln-forming activity involves the use of molds to form glass into more complex shapes. Virtually any shape that can be formed in clay or wax can also be made in glass. These more advanced kiln forming processes include kiln casting (melting glass into a mold inside a kiln, pate de verre (forming shapes by heating a “paste of glass” inside the kiln), and glass casting (pouring molten glass into a mold). These processes tend to be more complicated than basic fusing and slumping.

The Heating Phase

The “heating” phase, which takes place between room temperature and around 1200° to 1700° F (depending on the process you are performing), is where the glass makes the transition from a solid to a more flowing form. As glass is heated and moves through this phase, it goes through three separate states. First, from room temperature up to about 1000° F (540° C), glass remains rigid and brittle. It is expanding slowly, but will still crack or break if the temperature increases too rapidly. This kind of temperature related fracture is called “thermal shock.”

How rapid is rapid enough to cause thermal shock? The answer depends on several factors, but the most important are the thickness of the glass and the width of the piece of glass.

By the time the temperature of the glass gets above 1000° F, any glue, moisture, or surface contaminants have burned off. The glass begins to soften slightly and the surface of the glass will look glossy. Thermal shock will not occur at this temperature.

When the temperature reaches around 1300° to 1400° F, the glass gradually becomes soft enough to conform to a mold. It starts to glow a bright yellowish-red. The edges may soften and round and two pieces of glass that are touching will begin to stick together. This is the temperature range where slumping takes place.

If heating continues above 1330° F and moves toward 1500° F (820° C), the color of the glass deepens and becomes more red. Glass in this range has slumped completely and even starts to stretch out of shape.

Full fusing, the complete merging of two or more pieces of glass into one, takes place at around 1500° F. Above that temperature, glass becomes increasingly liquid. Kiln casting and pate de verre take place in this range.

As the temperature moves above 1500° F, glass also glows bright red. Bubbles may move toward the surface of the glass and pop. By the time the temperature reaches around 1700° F (925° C), the glass is buttery and can be moved when prodded with a tool. The technique of manipulating molten glass with a tool is called “combing” or “raking.” Glass manipulation techniques should be undertaken with care and only after you have some experience with fusing and slumping.

The Soaking Phase

The “Soaking” phase generally occurs at the highest temperature in the cycle. This temperature is around 1500° F for fusing or around 1200° to 1300° F for slumping, but it can be higher or lower for different processes such as fire polishing, combing, or casting. The length of the soak time can also vary.

When slumping, longer soak times cause the glass to conform more closely to the mold. When fusing, longer soak times cause the piece to become flatter and smoother. How long to soak also depends on other factors, such as type of glass, the thickness of the glass, the final shape desired, and how long the kiln has taken to make it through the heating phase. Soaking can last as short as a minute or as long as an hour or more.