Beginning in the 15th century, Venetians created fine rock crystal known as cristallo. Colored glasses and techniques for decoration, such as gilding, also were developed. From Venice, the product was exported to Europe and the Byzantine Empire. Venetian glass trade fell after Constantinople was captured by the Turks in 1453 as glassmakers emigrated from Venice and established glass houses throughout Europe. Germans put their efforts into the preparation and selection of newer, purer raw materials. The quality of glass was improved by the addition of low-iron potash and purer quartz. By 1680, Bohemian “crystal”—basically a potash-lime glass—was developed. Previously, in 1674, George Ravenscroft of London developed “flint” glass, a lead-crystal composition made with a large proportion of calcined flints and potash. Using 15 percent lead oxide, quartz pebbles imported from the Po River in Italy, and purer potash, Ravenscroft produced a fine, lustrous glass, soft enough to be cut and engraved easily and with a greater refractive power than soda-lime glass. Before the year 1700, eleven houses in London were producing leaded crystal. Around this same time, Johann Kunckel in Germany developed a reliable formula for producing ruby-red glass using gold chloride by dissolving gold in aqua regia and mixed with the batch, then melted, formed, and reheated to “strike” the precipitation of the ruby-red hue. And by adding burned bone or horn to the soda-lime batch, Kunckel was able to develop a phosphate opal glass, termed porcelain glass or Milchglas.