Flow blue was a type of transfer pottery produced by Staffordshire, England, potters beginning in about 1820. Sold mostly in the U.S. market, flow blue was similar to traditional blue-and-white pottery, except that the blue color was deliberately blurred, an effect achieved by adding a cup of lime or ammonia to the kiln during glazing.
English manufacturers of antique flow blue included Wedgwood, Johnson Brothers, Minton, Royal Doulton, and Swansea. Patterns ranged from Blue Danube to Idris to the classic Willow. As for the objects themselves, they included toilet wares and teapots, plates and platters, vases and garden seats, and even dog bowls.
One interesting subset of flow blue is the blue-marble effect. All-over patterns such as Lazuli lent itself to this look: When given the flow-blue treatment, the pattern would blur so that from afar the object resembled a piece of carved, blue-veined marble.
By World War I, U.S. potteries were producing most of the flow blue for the domestic market, causing English potters to exit the business, which had never been popular in the U.K. to begin with. The desirability of the ware waned in both countries between the wars, but interest picked up again in the U.S. in the 1960s. Because large amounts of 19th-century flow blue had been shipped to, or manufactured in, the U.S., flow blue remains fairly reasonable to collect.