Stained Glass | Can’t Sleep? Learn About Light And Color Art

Stained glass has transformed sunlight into art for more than a thousand years. This episode explores how colored glass is made, how artisans assemble intricate windows from thousands of individual pieces, and why churches, cathedrals, and public buildings have long used light itself as a storytelling medium. Along the way, you’ll hear about medieval workshops, famous stained-glass traditions, and the surprisingly complex techniques required to turn sand, metal oxides, and sunlight into enduring works of art.

It’s steady and consistent, with no whispering and no sudden changes, just enough to give your mind something to follow as you wind down. Happy sleeping!

Transcript

Welcome to the I Can't Sleep Podcast, Where I help you drift off one fact at a time. I'm your host Benjamin Boster, And today's episode is about stained glass. Stained glass refers to colored glass as a material or art and architectural works created from it. Although it is traditionally made in flat panels and used as windows, The creations of modern stained glass artists also include three-dimensional structures and sculptures. Modern vernacular usage has often extended the term stained glass to include domestic lead light and objet d'art created from glasswork. For example, In the famous Lambs of Louis Comfort Tiffany. As a material, Stained glass is glass that has been colored by adding metallic salts during its manufacture. It may then be further decorated in various ways. The colored glass may be crafted into a stained-glassed window, In which small pieces of glass are arranged to form patterns or pictures, Held together traditionally by strips of lead, Called cames or combs. And supported by a rigid frame. Painted details and yellow-colored silver stain are often used to enhance the design. The term stained glass is also applied to enameled glass, In which the colors have been painted onto the glass and then fused to the glass in a kiln. Stained glass, As an art and a craft, Requires artistic skill to conceive an appropriate and workable design, And the engineering skills to assemble a piece. A window must fit snugly into the space for which it is made, Must resist wind and rain, And also, Especially in the larger windows, Must support its own weight. Many large windows have withstood the test of time and remain sustainably intact since the late Middle Ages. And Western Europe together with illuminated manuscripts. They constitute a major form of medieval visual art to have survived to the present day. In this context, The purpose of a stained glass window is not to allow those within a building to seize the world outside, Or even primarily to admit light, But rather to control it. For this reason stained glass windows have been described as illuminated wall decorations. The design of a window may be abstract or figurative. May incorporate narratives drawn from the Bible, History, Or literature. May represent saints or patrons, Or use symbolic motifs, In particular, Armorial. Windows within a building may be thematic, For example. Within a church, Episodes from the life of Christ. Within a parliament building, Shields of the constituencies. Within a college hall, Figures representing the arts and sciences. Or within a home, Flora, Fauna, Or landscapes. During the late medieval period, Glass factories were set up where there was a ready supply of silica, The essential material for glass manufacture. Silica requires a very high temperature to meld, Something not all glass factories were able to achieve. Such materials as potash, Soda, And lead can be added to lower the melting temperature. Other substances, Such as lime, Are added to make the glass more stable. Glass is colored by adding metallic oxide powders or finely divided metals while it is in a molten state. Copper oxides produce green or bluish green, Cobalt makes deep blue, And gold produces wine red and violet glass. Much modern red glass is produced using copper, Which is less expensive than gold and gives a brighter, More vermilion shade of red. Glass colored while in the clay pot in the furnace is known as pot metal glass, As opposed to flashed glass, Which fuses a thin layer of colored glass to a thicker layer of colorless glass to produce the desired color. Using a blowpipe, A glassmaker will gather a glob of molten glass that was taken from the pod heating in the furnace. The gatherer is formed to the correct shape and a bubble of air blown into it. Using metal tools, Molds of wood that would have been soaking in water and gravity, The gatherer is manipulated to form a long cylindrical shape. As it cools, It is reheated so that the manipulation can continue. During the process, The bottom of the cylinder is removed. Once brought to the desired size, It is left to cool. One side of the cylinder is opened, And the cylinder is then put into another oven to quickly heat and flatten it. And then placed in an annealer to cool at a controlled rate. Making the material more stable. Hand-blown or mouth-blown cylinder, Also called muff glass, And crown glass, Were the types used in the traditional fabrication of stained glass windows. Crown glass is hand-blown glass created by blowing a bubble of air into a gather of molten glass, And then spinning it, Either by hand or on a table that revolves rapidly like a potter's wheel. The centrifugal force causes the molten bubble to open up and flatten. It can then be cut into small sheets. Glass formed this way can be either colored and used for stained glass windows, Or uncolored as seen in small paint windows in the 16th and 17th century houses. Concentric curving waves are characteristic of the process. The center of each piece of glass, Known as the bullseye, Is subject to less acceleration during spinning, So it remains thicker than the rest of the sheet. It also has the pontil mark, A distinctive lump of glass left by the pontil rod, Which holds the glass as it is spun out. The lumpy, Refractive quality means the bullseyes are less transparent. But they have still been used for windows, Both domestic and ecclesiastical. Crown glass is still made today, But not on a large scale. Rolled glass, Sometimes called table glass, Is produced by pouring molten glass onto a metal or graphite table. And immediately rolling it into a sheet using a large metal cylinder, Similar to rolling out a pie crust. The rolling can be done by hand or by machine. Glass can be double rolled, Which means it is passed through two cylinders at once, Similar to the clothes ringers on older washing machines, To yield glass of a specified thickness, Typically about 1⁸ or 3 mm. The glass is then annealed. Rolled glass was first commercially produced around the mid-1830s and is widely used today. It is often called cathedral glass, But this has nothing to do with medieval cathedrals, Where the glass used was hand-blown. Architectural glass must be at least an eighth of an inch thick to survive the push and pull of typical wind loads. However, In the creation of red glass, The colouring ingredients must be of a certain concentration, Or the colour will not develop. This results in a color so intense that at the thickness of an eighth inch, The red glass transmits little light and appears black. The method employed to create red stained glass is to laminate a thin layer of red glass to a thicker body of glass that is clear or lightly tinted, Forming flashed glass. A lightly colored molten gather is dipped into a pod of molten red glass, Which is then blown into a sheet of laminated glass using either the cylinder, Muff, Or the crown technique. Once this method was found for making red glass, Other colors were made this way as well. A great advantage is that the double-layered glass can be engraved or abraded to reveal the clear or tinted glass below. The method allows rich detailing and patterns to be achieved without needing to add more lead lines, Giving artists greater freedom in their designs. A number of artists have embraced the possibilities flash glass gives them. For instance, 16th century heraldic windows relied heavily on a variety of flash colors for their intricate crests and creatures. In the medieval period, The glass was abraded. Later, Hydrofluoric acid was used to remove the flash in a chemical reaction. And in the 19th century, Sandblasting started to be used for this purpose. Islamic civilization played a major role in inspiring the art of stained glass from the 8th century onward. Mosques, Homes, And cities were transformed into beautiful spaces decorated with glass. Beauty and function were essential elements of design in Islamic civilization. Perhaps in an effort to supply thousands of mosques, But also thanks to the input provided by thriving scientific activity in fields such as optics and chemistry. Islamic glassmakers transformed what had previously been a craft into Islamic stained glass, An industry employing new technologies and a large workforce from across Islamic civilization. Across Islamic civilization, Glass vessels were mass-produced from the 8th century onwards, Either by blowing liquid glass in chambers, Or by cutting it from crystal. Glassmakers in Syria, And to a lesser extent Egypt, Inherited and improved upon this glassmaking technique, Developing their own techniques for mastering the art of Islamic stained glass, Its coloring, And its decoration, Expanding the variety of products. The history of stained glass in Syria dates back to ancient times, As Syria was influenced by successive civilizations, Such as the Roman and Byzantine. Syrian stained glass is characterized by its intricate geometric and floral designs, Reflecting the skill of Syrian artisans. There are a number of glass factories, Particularly in Germany, The United States, England, France, Poland, Russia, And Syria, Which produce high-quality glass, Both hand-blown, Cylinder, Disc, And crown, And rolled, Cathedral, And opalescent. Contemporary stained glass artists have a number of resources to draw on in their work for centuries from other artists from whom they learn, Continuing the tradition in new ways. In the late 19th and 20th centuries, There were many innovations and techniques and types of glass used. Many new types of glass were developed for use in stained glass windows, Notably Tiffany glass and stained glass panels. The primary method of including color in stained glass is to use glass, Originally colorless, That has been given coloring by mixing with metal oxides in its melted state in a crucible or pot, Producing glass sheets that are colored all the way through. These are known as pot metal glass. A second method, Sometimes used in some areas of Windows, Is flashed glass. A thin coating of colored glass, Used to colorless glass, Or colored glass, To produce a different color. In medieval times, Glass flashing was especially used for reds, As glass made with gold compounds was very expensive, And tended to be too deep in color to use at full thickness. Another group of techniques give additional coloring, Including lines and shading, By treating the surfaces of the colored sheets and often fixing these effects by a light firing in a furnace or kiln. These methods may be used over broad areas, Especially with silver stain, Which gave better yellows than other methods in the Middle Ages. Alternatively, They may be used for painting linear effects or polychrome areas of detail. The most common method of adding the black linear painting necessary to define stained glass images is use of what is variously called glass paint, Vitreous paint, Or grisaille paint. This was applied as a mixture of powdered glass, Iron or rust fillings to give a black color, Clay and oil, Vinegar or water for a brushable texture with a binder such as gum arabic. This was painted on the pieces of colored glass and then fired to burn away the ingredients giving texture, Leaving a layer of the glass and coloring fused to the main glass piece. Silver stain, Introduced soon after 1300, Produced a wide range of yellow to orange colors. This is the stain in the term stained glass. Silver compounds are mixed with binding substances, Applied to the surface of glass, And then fired in a furnace or kiln. They can produce a range of colors, From orange-red to yellow. Used on blue glass, They produce greens. The way the glass is heated and cooled can significantly affect the colors produced by these compounds. The chemistry involved is complex and not well understood. The chemicals actually penetrate the glass they are added to a little way, And the technique therefore gives extremely stable results. By the 15th century, It had become cheaper than using pot metal glass, And was often used with glass paint as the only color on transparent glass. Silver stain was applied to the opposite face of the glass to silver paint, As the techniques did not work well one on top of the other. The stain was usually on the exterior face, Where it appears to have given the glass some protection against weathering, Although this can also be true for paint. They were also probably fired separately, The stain needing a lower heat than the paint. Sanguine Carnation Rouge Jean Cousin, Or Cousin's Rose, After its supposed inventor, Is an iron-based fired paint producing red colors, Mainly used to highlight small areas, Often on flesh. It was introduced around 1500. Copper stain, Similar to silver stain, But using copper compounds, Also produced reds, And was mainly used in the 18th and 19th centuries. Cold paint is various types of paint that were applied without firing. Contrary to the optimistic claims of the 12th century writer Theophilus Presbyter, Cold paint is not very durable, And very little medieval paint has survived. As well as painting, Scratch-graffito techniques were often used. This involved painting a color over pot metal glass of another color, And then before firing, Selectively scratching the glass paint away to make the design, Or the lettering of an inscription. This was the most common method of making inscriptions in early medieval glass, Giving wide or light letters on a black background. With later inscriptions more often using black painted letters on a transparent glass background. Ordinary soda-lime glass appears colorless to the naked eye when it is thin, Although iron oxide impurities produce a green tint, Which becomes evident in thick pieces or with the aid of scientific instruments. A number of additives are used to reduce the green tint, Particularly if the glass is to be used for plain window glass rather than stained glass windows. These additives include manganese dioxide, Which produces sodium permanganate and may result in a slightly mauve tint, Characteristic of the glass in older houses in New England. Selenium has been used for the same purpose. While very pale green is the typical color of transparent glass, Deeper greens can be achieved by the addition of iron to oxide, Which results in a bluish green glass. Together with chromium, It gives glass of a richer green color, Typical of the glass used to make wine bottles. The addition of chromium yields dark green glass, Suitable for flashed glass. Together with tin oxide and arsenic, It yields emerald green glass. In medieval times, Blue glass was made by adding cobalt blue, Which at a concentration of 0. 025% to 0. 1% in sodalime glass, Achieves the brilliant blue characteristic of Chartres Cathedral. The addition of sulfur to boron-rich borosilicate glasses imparts a blue color. The addition of copper oxide at 2-3% produces a turquoise color. The addition of nickel at different concentrations produces blue, Violet, Or black glass. Metallic gold in very low concentrations, Around 0. 001%, Produces a rich ruby-colored glass. Ruby gold. In even lower concentrations it produces a less intense red, Often marketed as cranberry glass. The color is caused by the size and dispersion of gold particles. Ruby gold glass is usually made of lead glass with tin added. Pure metallic copper produces a very dark red opaque glass. Glass created in this manner is generally flashed. It was used extensively in the late 19th and early 20th centuries, And exploited for the decorative effects that could be achieved by sanding and engraving. Selenium is an important agent to make pink and red glass. When used together with cadmium sulfide, It yields a brilliant red color known as selenium ruby. Yellow glass was very often achieved by silver stain applied externally to the sheets of glass. The addition of sulfur together with carbon and iron salts is used to form iron polysulfides and produce amber glass ranging from yellowish to almost black. With calcium yields a deep yellow color. Adding titanium produces yellowish-brown glass. Titanium is rarely used on its own, And is more often employed to intensify and brighten other additives. Cadmium, Together with sulfur, Results in a deep yellow color, Often used in glazes. However, Cadmium is toxic. Uranium can be added to give glass a fluorescent yellow or green color. Uranium glass is typically not radioactive enough to be dangerous. But if ground into a powder, Such as by polishing with sandpaper and inhaled, It can be carcinogenic. When used with lead glass with a very high proportion of lead, It produces a deep red color. The addition of manganese gives an amethyst color. Manganese is one of the oldest glass additives, And purple manganese glass has been used since early Egyptian history. Nickel, Depending on the concentration, Produces blue or violet, Or even black glass. Lead crystal with added nickel acquires a purplish color. Tin oxide with antimony and arsenic oxides produce an opaque white glass, First used in Venice to produce an imitation porcelain. White glass was used extensively by Louis Comfort Tiffany to create a range of opalescent, Mottled, And streaky glasses. The first stage in the production of a window is to make or acquire from the architect or owners of the building an accurate template of the window opening that the glass is to fit. The subject matter of the window is determined to suit the location, A particular theme, Or the wishes of the patron. A small design called a vidimus, From Latin, We have seen, Is prepared, Which can be shown to the patron. A scaled model maquette may also be provided. A designer must take into account the design, The structure of the window, The nature and size of a glass available, And their own preferred technique. A traditional narrative window has panels which relate a story. A figurative window could have rows of saints or dignitaries. Scriptural texts or mottos are sometimes included, And perhaps the names of the patrons or the person to whose memory the window is dedicated. In a window of a traditional type, It is usually left to the discretion of the designer to fill the surrounding areas with borders, Floral motifs, And canopies. A full-size cartoon is drawn for every light opening of the window. A small church window might typically have two lights, With some simple tracery lights above. A large window might have four or five lights. The east or west window of a large cathedral might have seven lights in three tiers, With elaborate tracery. In medieval times, The cartoon was drawn directly on the surface of a whitewashed table, Which was then used as a pattern for cutting, Painting, And assembling the window. The cartoon is then divided into a patchwork, Providing a template for each small glass piece. The exact position of the lead which holds the glass in place is also noted, As it is part of the calculated visual effect. Each piece of glass is selected for the desired color and cut to match a section of the template. An exact fit is ensured by grossing the edges with a tool which can nibble off small pieces. Details of faces, Hair, And hands can be painted onto the inner surface of the glass using a special glass paint which contains finely ground lead or copper fillings, Ground glass, Gum arabic, And a medium such as wine or vinegar. The art of painting details became increasingly elaborate and reached its height in the early 20th century. From 1300 onwards, Artists started using silver stain, Which was made with silver nitrate. It gave a yellow effect ranging from pale lemon to deep orange. It was usually painted onto the outside of a piece of glass, Then fired to make it permanent. This yellow was particularly useful for enhancing borders, Canopies, And halos, And turning blue glass into green glass. By about 1450, A stain known as Cousin's Rose was used to enhance flesh tones. In the 16th century, A range of glass stains were introduced, Most of them colored by ground glass particles. They were a form of enameled glass. Painting on glass with these stains was initially used for small heraldic designs and other details. By the 17th century, A style of stained glass had evolved and was no longer dependent upon the skillful cutting of colored glass into sections. Scenes were painted under glass panels of square format, Like tiles. The colors were then annealed to the glass before the pieces were assembled. A method used for embellishment and gilding is the decoration of one side of each of two pieces of thin glass, Which is then placed back to back within the lead came. This allows for the use of the techniques such as angle gilding and aglomizé to produce an effect visible from both sides, But not exposing the decorated surface to the atmosphere or mechanical damage. Once the glass is cut and painted, The pieces are assembled by slotting them into H-sectioned lead cams. All the joints are then soldered together, And the glass pieces are prevented from rattling, And the window made weatherproof by forcing a soft, Oily cement or mastic between the glass and the cams. In modern windows, Copper foil is now sometimes used instead of lead. Traditionally, When a window was inserted into the window space, Iron rods were put across it at various points to support its weight. The window is tied to these rods with lead strips, Or, More recently with copper wires. Some very large early Gothic windows are divided into sections by heavy metal frames, Called ferramenta. This method of support was also favored for large, Usually painted windows of the Baroque period.