According to historians, artisans of the past would use a color-by-numbers technique to design a life-size sketch of the window, then cut and assemble the glass pieces together into a lead frame. Bubbles in the glass caused the light to dance to a stunning effect indoors.
You might say that an appreciation for stained glass is as timeless as the art form itself. Windows with colorful mosaic patterns were once revered for casting beautiful rainbows over church-goers in the Middle Ages.
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.
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.