This is an early silver bowl incandescent lamp made by GE-Mazda of America - possibly one of the first as it has test numbers written on the base and was obtained with many other early reflector lamps from a retired NELA Park engineer.
Silver Bowl incandescent lamps were invented in 1933 by the Silvray company of New York, and quickly set a new trend in glare-free indirect lighting of indoor spaces. Silvray was not a lampmaker, but established the first durable and cost-effective process of applying mirror coatings to lamps. These were applied to the neck, bowl, or side of ordinary pear-shaped lamps. The company marketed a successful line of fixtures based on the silver bowl lamps, in which they were suspended base-up to floodlight the ceiling. Owing to the complete obscuration of the filament from view, a room could be indirectly lit with very low glare. They were also used in white enamel RLM type reflector fixtures in high powers up to 1000W.
The coating pre-dates modern aluminised reflector lamps and consists of a chemically-desposited mirror of highly reflective pure silver applied to the outer surface of the glass. To prevent degradation it is then sealed under an electroplated layer of copper. The resulting coating is very fragile, and scratch resistance is improved with a top coat of durable aluminium flakes applied by a painting process. The luminous flux is approx. 7% lower than uncoated lamps.
Although the main lampmakers like GE-Mazda soon offered silver bowl lamps, these were all coated by Silvray owing to that company's strong patent situation and refusal to license its process. These early silver bowl lamps were very expensive, costing around three times the price of uncoated lamps. They were superseded by a lower cost version based on an internal aluminium coating soon after GE had developed the vacuum metallizing process in 1936, in which an aluminium mirror is evporated onto the glass under high vacuum.