Forming a falling curtain of molten glass

A free-falling curtain of molten glass that is uniform in thickness and temperature is formed by flowing the glass over the lip of a forehearth after conditioning the glass in said forehearth. The forehearth is constructed to provide an increasing rate of fall as the glass travels, flows from the furnace through the forehearth and progressively cools. The floor of the forehearth is essentially the mirror image of the viscosity versus temperature curve of the material being treated. Other molten materials that become viscous as they cool can also be treated by the method of my invention to provide a uniform falling curtain of said material.

BACKGROUND OF THE INVENTION 
This invention relates to the formation of a falling curtain of molten 
silicate glass that is required in the production of silicate glass flakes 
and/or fibers by atomization of said falling curtain. More particularly, 
this invention provides an improvement in said process by providing a 
falling curtain that is uniform in temperature and thickness. 
U.S. Pat. Nos. 3,794,475 and 3,840,359, to Lazet, teach methods of forming 
flakes or fibers of soluble silicate glass. Molten glass is formed into a 
falling curtain which is fragmented into flakes or fibers by a stream of 
high pressure gas or liquid. The production of high yields of uniformly 
sized flakes or fibers by these processes has been difficult. The most 
important factor involved in these preparations appear to be the viscosity 
of the molten glass and the uniformity of the falling curtain. 
It is an object of this invention to provide a falling curtain of silicate 
glass which is of uniform thickness and temperature. It is a further 
object of this invention to provide a forehearth which promotes a flow of 
molten glass at a uniform depth as the glass is cooled to the temperature 
required for the formation of fibers and flakes. 
SUMMARY OF THE INVENTION 
A uniform falling curtain of molten material that becomes viscous as it 
cools can be formed by conditioning said material in a forehearth. In 
order to provide a falling curtain which is uniform as to temperature and 
thickness, the glass must be maintained at a uniform depth while being 
conditioned in the forehearth. I have found that molten glass can flow 
through a forehearth at a uniform depth if the rate of fall or pitch of 
the floor in the forehearth increases in relation to the rate of glass 
cooling. 
THE INVENTION 
The success of the processes taught in U.S. Pat. Nos. 3,794,475 and 
3,840,359, hereby incorporated by reference, is dependent upon the 
formation of a falling curtain of molten glass which is uniform in 
thickness and in temperature. The soluble silicate glasses employed in 
these processes are prepared in a furnace of the open hearth type of 
fusing a source of alkali metal, usually sodium carbonate and a source of 
silica, usually sand. The fusion is carried out at a temperature above 
2000.degree. F therefore, the glass when drawn is above 2000.degree. F. 
The temperature required for atomization is below about 1900.degree. F so 
that the glass must be cooled. The glass is drawn into a forehearth and 
cooled as it flows from the furnace. Upon flowing over the forehearth lip, 
a falling curtain is formed. Several factors prevent the formation of the 
falling curtain in a uniform thickness and temperature. The glass near the 
edges of the forehearth cools more rapidly than that near the center, so 
that the falling curtain will be non-uniform in temperature. The higher 
viscosity of the cooler glass will also cause the falling curtain to be 
thicker near the edges. Additionally, the thickness of the falling curtain 
should be controlled to about one inch or less. This control is difficult 
to achieve because of the viscosity increase as the glass cools. The 
increasing drag of the cooling glass causes the initially shallow stream 
to become deeper and less uniform. 
I have found that a uniform falling curtain of molten glass can be formed 
by conditioning the glass in a forehearth constructed in such a manner 
that the level of the glass remains the same as it flows through the 
device. The forehearth is constructed with a compound slope or rate of 
fall. The rate of fall should be directly proportioned to the viscosity of 
the material being treated or inversely proportioned to the temperature. I 
prefer that the slope of the forehearth be such that its floor 
approximates the mirror image of the temperature versus viscosity curve 
for the material being treated. This curve is drawn with the temperature 
as the abcissa and the viscosity as the ordinate. The glass is maintained 
at a uniform temperature across the forehearth by the use of sidewall 
burners to ameliorate the heat loss of the glass flowing near the walls. 
By observing these practices, a uniform flow of glass can be cooled in the 
forehearth to provide the desired results. 
In some cases, it is desired that the depth of the glass stream as it flows 
over the forehearth lip be somewhat less than the depth throughout the 
remainder of the device. To achieve this reduction in depth, the last 
section of the forehearth is constructed with an even greater pitch so 
that the speed of the glass flow is increased and the depth reduced.

EXAMPLES 
The following Examples illustrate certain embodiments of my invention and 
certain aspects of the prior art. 
EXAMPLE 1 
This Example illustrates the prior art of using a forehearth with a 
constant slope. The forehearth was 20 feet long and had a constant slope 
of 3/4 inch per foot. Sodium silicate glass having a composition of 2.37 
moles of SiO.sub.2 per mole of Na.sub.2 O was prepared in a conventional 
glass furnace at a temperature of 2000.degree. F. The molten glass was 
drawn into the forehearth at the rate of 2500 lbs./hr. and the level of 
the molten glass was 1/2 inch as drawn from the furnace. The depth was 
over 2 inches near the forehearth lips and the depth was not uniform 
across the width of the forehearth. The falling curtain provided was not 
uniform and when atomized, gave rise to a number of products including 
large globs of glass, some fibers and a few flakes. 
EXAMPLE 2 
This Example illustrates the process of my invention and the advantages it 
provides. In this case, the 20 ft. forehearth was divided into 5 four ft. 
sections. The rate of fall for each zone was: 
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RATE OF FALL 
ZONE B (INCHES PER FOOT) 
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1 1/8 
2 3/8 
3 5/8 
4 7/8 
5 11/8 
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Zone 1 is nearest the furnace while zone 5 is at the end of the forehearth. 
The glass prepared as described in Example 1 was drawn into the forehearth 
which was equipped with sidewall burners. The initial depth of the glass 
stream was 1 inch and it could be seen that the depth of the glass did not 
increase and that the flow was uniform across the forehearth. The glass 
flowed over the forehearth lip to form a uniform falling curtain. 
EXAMPLE 3 
The process of Example 2 was repeated except that the forehearth was 
altered so that zones 4 and 5 had rates of fall of 1 inch per ft. and 11/2 
inches per ft., respectively. In this case, the depth of the stream 
flowing over the forehearth lip was 1/2 inch. The uniform falling curtain 
was atomized as desired to provide flakes or fibers depending upon the 
temperature of the glass.