Osmalto, or smalt for short, has a technical definition of being crushed or powdered glass enamel, primarily blue and has its roots in the jewelry trade. In cloisonné work, it was applied to surface designs sectioned off by thin strips of metal and fired in a kiln until melted. You often see small trinket boxes decorated in this manner.

The popular use in our trade began in the mid 1870’s, coinciding with the birth of carved wooden letters. For maximum durability and visibility, these letters were without doubt gold leafed, and projected their own elegance.

Requiring a worthy background, it helps to understand the nature and practice of sign construction during this era. Plywood wasn’t yet invented so if the customer desired a mid-ranged sign, you nailed up a wooden framework and covered it with a tin face. This gave you a smooth working surface and required a minimum of effort. For the highest quality and weather resistance of sign boards, wooden planks were dowelled together and to prevent any seams or cracks from ever showing, duck canvas was varnish affixed to the face. After initial sealing and priming, it would be wasted effort to build up its rough texture and if accomplished, wouldn’t appear any nicer than its cheaper tin covered counterpart. There was considerable cost involved in hand carved and gilded letters, so longevity was the main factor. The use of colored glass smalt was a natural conclusion. It not only disguised the rough canvas with minimum effort, it imparted an elegant glassy texture with durability and fade resistance bar none. I have seen existing exterior examples on the east coast still attractive after a century of use. Longevity aside, it’s a simple solution to the coarse or boring. The era of beautifully grained sandblasted wood signs seems to be giving way to prismatic letter faced, computer routed urethane panels with perfectly smooth backgrounds just screaming for attention.

Smalt is divided into two categories, glass and sand. The latter is simply the beach variety traditionally coated with japan colors (see Atkinson’s Sign Painting or Gold Leaf Techniques, 4th Edition.) It has a nice dull to satin sheen to it but because of its paint finish, is best reserved for interior use. Although not readily available in its original supplied form, there is an extremely durable epoxy coated version manufactured by the Clifford W. Estes Co. of Lyndhurst, NJ (an original maker of smalts in the last century) referred to as masonry aggregate and available in many colors.

The most commonly used is glass, for its shiny appearance and light fastness in exterior use. The most widely used is of coffee ground size and comes in a range of colors from jet black and ruby red to cobalt blue and dark green. Being most always used with gilded graphics, the darkest colors are preferred for maximum contrast.

In order to attach the smalts to a surface a SMALT PAINT is required. Although many materials have been used in recent years with varied results, keep in mind that merely sticking them to the surface will give you bald spots with just short service. They must sink into the adhesive a third to one half of their thickness to be permanently adhered. There is also the matter of open time when dealing with larger signs. The original recipe was to take keg white lead (which was of paste thickness), break it down with additional linseed oil to a brushing consistency and add some color to it to closely match the smalt.

In keeping with its character, our modern method is to use a half and half mix of (1) a lettering enamel matching the smalt (black with black, brilliant blue with cobalt, etc) and (2) a conditioning cream made for bulletin board pictorial work. Smith’s Cream by Dana, Jack’s Cream by Ronan and Jones Cream by Akzo are the brands available. Mixing the enamel & cream together 50/50 will give you the viscosity needed to fix the smalt, and the open time sufficient to easily do a twenty foot sign with its drying time just overnight. Some craftsmen use a product called Hol-Fast oil or epoxies for small areas but I’m assured the open time is minimal.

For application, first prepare a smalting can by taking a coffee container and perforating the bottom overall with 1/8” holes using and awl or hammer and nail. Cover the bottom with a piece of pasteboard and fill the can. When the board is removed you are able to evenly shake and distribute the smalt evenly. This type of background isn’t limited to raised letters or routed recesses but is just as handsome with flat surface gilded inscriptions. Lay your panel flat on a horizontal bench or saw horses. With recessed or flat gilded work apply the paint thickly, brushing from dry to wet where you can until evenly covered. On larger or very long panels you can paint in a certain distance, say a third of the area or so, apply the smalt heavily and evenly within 8 or 10 inches of the wet end and continue with the paint where you left off. Your own judgment will dictate how much you do at once. As a rough rule of thumb, one pound of smalt will cover two square feet of area. You needn’t wait until you’ve completely covered the sign with smalt before you dump off the excess. If you’re running short of the cheaper material (black glass or sand) or using the much more expensive colored glass, periodically flip the panel over rapidly with a tarp on the floor. This catches the excess which you reuse.

(Thanks to John Jordan for supplying this document to this site, and also to Kimberly Zanetti for retyping the old document for use here!)