Not long after Henri Becquerel discovered radioactivity in uranium salts, Madame Curie discovered both polonium and radium as two new radioactive elements also present with uranium. The relatively high specific activity and moderate half-life of 1,600 years of Ra-226, the main radioisotope of radium found in uranium ore, made for a material which when mixed with a phosphor allowed for a glow-in-the-dark substance.
Thus, in addition to its medical usage, radium usage also became a major industry in the 1920s and 1930s for making watch, clock and aircraft dials. The radium dial painters brought a certain degree of notoriety to the abuse of radioactive materials, and that precautions needed to be followed with this new substance.
Because it takes approximately three metric tons of uranium to extract 1 gram of Ra-226, prodigious quantities of uranium were mined to sustain this new industry. The uranium ore itself was a "waste product" of this industry. By some estimates, nearly one million tons of uranium were mined to support this industry.
Taking advantage of this newly abundant resource, the tile and pottery glazing industry then had a relatively inexpensive and abundant source of glazing material that produced a wide variety of colors depending upon admixtures, firing, etc.
Vibrant colors of orange, yellow, red, green, blue, black, mauve, etc. were produced on tiles and other ceramic materials, and by some estimates, some 25% of all houses and apartments constructed during that period [circa 1920–1940] used varying amounts of bathroom or kitchen tiles that had been glazed with varying amounts of uranium. These can now be readily found in older homes, apartments, and other buildings still standing from that era by use of a simple geiger counter that readily detects the beta radiation emitted by uranium's ever-present decay chain radio-daughters.
In most situations, the radiation exposure is not excessive, but there are some exceptions in which pure uranium oxide [which produces red-orange coloration as a glaze] on bathroom floors can pose a hazard for infants crawling around for hours on end, day after day.
- http://www.nbcnews.com/id/3077213/ns/technology_and_science-science/t/uranium-hunter-follows-trail-tiles/ Uranium hunter follows trail of tiles