Patent GB-746614-A (GB)
Title: Coated illuminating glassware and method of manufacture thereof
IPC: H01J5/08
Filed: 1954-01-29
Abstract: For producing a light-diffusing coating on a light-transmitting glass sheet or upon the inner surface of a glass lamp bulb, the surface of the glass is contacted or flushed with a slightly thixotropic aqueous suspension of a finely divided amorphous mixture comprising essentially SiO2 and, per mol of SiO2, about 0.005 to 0.7 mol on the oxide basis of the metal-containing reaction product resulting from the reaction, in an aqueous medium, of ammonia and a water-soluble compound selected from the group consisting of water-soluble compounds of magnesium, calcium, strontium, and barium, and mixtures of such compounds, the suspension containing 0.85 to 1.5 gram mols of SiO2 per litre of water and sufficient ammonium hydroxide to have a pH value between 7.3 and 12, and a viscosity immediately after agitation of about 7.5 to 12 centistokes at 25 DEG C., the excess suspension being allowed to drain from the body, and the adherent suspension dried rapidly.  The coating may be applied to clear or frosted bulbs, and the applied suspension may include coloured or fluorescent pigments.  Further, the coating may be employed to diffuse both visible radiation and ultra-violet, for example on the inside surface of the outer envelope of a high pressure mercury arc lamp.  The silica that is used consists of a washed filter cake prepared by precipitation from ammonium silicofluoride in aqueous solution by reaction with ammonium hydroxide, a strong aqueous solution of ammonium silicofluoride being poured into the ammonia solution, each at a temperature of 25-37 DEG C. and the solution mixed as rapidly as possible.  The precipitated silica is filtered and washed to remove ammonium fluoride and excess ammonia, since as little as 1.5 per cent of the former in the filter cake will tend to shorten the filament life of a lamp if embodied in the coating on the interior of the bulb.  In an example 330 c.c. of tap water was added to a portion of wet filter cake containing about 203 c.c. of water and 35 grams of SiO2 in a container, and the silica dispersed by stirring.  To the suspension were added 0.25 c.c. of 85 per cent H3PO4 and 9 c.c. of an aqueous solution of magnesium and calcium formates containing 0.026 g. of MgO and 0.026 g. of CaO per c.c., and the mixture was milled in a colloid mill set with a clearance of 0.003 inch.  The pH value then amounted to 8, and the viscosity, measured at 25 DEG C. immediately after agitation, was 7.7 centistokes.  This suspension applied to an inside frosted bulb and dried for 3 minutes at 500 DEG C., gave an adherent, uniform coating, with a transmission of about 95 per cent, and hid the filament completely.  To produce a coloured coating a colouring pigment or compound free from constituents likely to damage a lamp bulb may be added to the silica suspension before the latter is passed through the colloid mill, the pH value of the suspension being preferably adjusted to the figure 8 by the addition of ammonium hydroxide or an acid such as formic acid, dependent on the direction of adjustment necessary.  Examples of compounds suitable for providing coatings giving a pink or flesh colouring, blue, lavender, yellow, green, &c., are given in the Specification.  Phosphors which convert light of low luminosity, such as ultra violet radiation, to light of high luminosity, may be included to increase the efficiency of a lamp.  By utilising a phosphor having a maximum emission of a specific wave length a lamp which emits light of a specific colour can be produced. Specification 380,464 [Group XVI] is referred to.