From the studio

A guide to stained glass mosaic

Stained glass mosaic sits between two old crafts: the leaded window and the tesserae mosaic. This guide walks through how we approach the material at Tom's Gallery — from choosing a sheet of glass to lighting a finished panel from behind.

Choosing the glass

The character of a stained glass mosaic is decided at the sheet. Cathedral glass is transparent and holds saturated colour when lit from behind. Opalescent glass is milky and diffuses light softly, which is useful for skin tones or overcast skies. Wispy and streaky sheets combine the two and give a painterly base without any painting. Textured surfaces — hammered, granite, seedy — scatter light and add depth to backlit work.

For a first panel, pick three or four sheets that already look good side by side on the light table. A mosaic will rarely rescue colours that fight in the raw sheet.

Tools and safety

A pistol-grip glass cutter with a self-oiling head, running pliers, grozing pliers, wheeled mosaic nippers, and a small electric glass grinder cover almost every cut. Work over a felted board so shards do not skate. Wear closed shoes, safety glasses, and a mask when grinding — glass dust is fine and sharp.

Cartoon and layout

The cartoon is the full-size drawing under the mosaic. Keep lines thick enough to read through glass, and mark the direction of the "andamento" — the flow of the tesserae — with arrows. Andamento is what turns a field of pieces into a moving surface: pieces radiate from a face, follow the muscle of an animal, or ripple around a focal shape. Cut a few test tesserae and lay them dry on the cartoon before committing.

Cutting tesserae

Score once, in one confident pass, with the cutter held perpendicular to the sheet. Break the score immediately with running pliers — a cold score loses its edge. For curves, score a series of short tangent lines and nibble the waste away with wheeled nippers. A grinder cleans the final edge and lets neighbouring pieces sit within a millimetre of each other, which is what a tight mosaic needs.

Substrate and adhesive

For a backlit panel the substrate must transmit light. Clear tempered glass or cast acrylic both work; acrylic is lighter and forgiving to drill. Bond tesserae with a UV-stable, water-clear silicone or a two-part epoxy specified for glass. Avoid PVA and standard mastic — they yellow under an LED and telegraph every brushstroke once the light is on.

Grouting — or not

Traditional cementitious grout gives a matte, painterly finish and unifies the surface, but it blocks light at every joint. For backlit work we usually leave the joints open, or fill them with a translucent epoxy tinted in a neutral tone so the glow reads as one continuous field. If you do grout, seal the glass edges first with a thin coat of the same adhesive so pigment does not creep under the tesserae.

Lighting the panel

A shallow shadow box with a diffusing sheet behind the mosaic gives the most even light. Use high-CRI LED strips (CRI 95+) at around 3000K for warm work or 4000K when the palette is cool — cheaper strips shift the greens and reds unpredictably. Leave 30–50 mm between the LED and the diffuser so individual diodes do not print through as bright dots.

Care

Dust with a soft brush; clean with a microfibre cloth barely damp with distilled water. Avoid ammonia, which fogs some art glass, and do not press hard on grouted joints. Hung out of direct sun, a stained glass mosaic will outlast the wall it hangs on.