Paul Catapano asks:

What was/is the Phil. Quartz Co.?

Philadelphia Quartz Co. was apparently the main supplier of silicate of
soda in the eastern U.S. PQ had a sizeable tank car fleet at least through
the 1960s, and also leased cars from GATX and SHPX. They had a west coast
subsidiary as well, Philadelphia Quartz Co. of Calif., which also operated
a dozen or so tank cars.

What are "Soluble Silicates"?

Scott Chatfield has partly answered this; in addition to the uses he
mentions, silicate of soda was used in making glass and steel and for other
industrial purposes, so there was a sizeable demand for it in tank car size
quantities.

Also, FWIW, the 1951 date was a reweigh date, probably at the time the car
was equipped with AB air brakes; the Type 21 tank cars were actually built
in the 1920s, and if you look carefully at the data on the right end of the
tank you will find the car's actual built date, based on an AC&F builder's
photo. I can say this with confidence as I researched the lettering scheme
for that model (as well as the other P2K tank car models).

Richard H. Hendrickson
Ashland, Oregon 97520