How to identify old ironstone
Early ironstone, or stoneware, was originally made in England and was meant to be an alternative to the more fragile earthenware and porcelain. Charles Mason and his family patented ironstone in 1813. By 1870, American potteries were making white "graniteware." Old white ironstone is still relatively easy to find.
Old blue ironstone is scarcer.
Use tactile and visual judgment.
Old ironstone may or may not have hand-painted designs or transfers. It may also have a raised design. It will always be opaque. Under light, you should not see any translucence. As you hold it, you will notice that compared to other dishware, it is quite heavy. Lift a piece of known ironstone and compare how it feels with a similar piece that is not ironstone.
While new and early 20th-century ironstone comes in many colours, old ironstone is usually creamy white (American) or white with a very slight blue cast (English). Though rarer, you may also find beautiful examples of blue ironstone. The more primitive-looking jugs may have a beige-grey background with or without a cobalt blue painted flower or bird. Fancier pieces may have faded-looking light blue backgrounds with cobalt blue pictorials.
Identify and date old ironstone by register marks on the bottom of the piece. Spode, Wedgewood, J&G Meakin, Wood & Sons, Birks Bros. & Seddon, T & R Boote, Turner & Tomkinson as well as other companies all made English ironstone. A great deal of English ironstone was exported to the United States. American ironstone makers included Red Cliff, Knowels, Taylor & Knowels, W.A. Lewis (Galesville NY), and McCoy. Be aware that McCoy was manufacturing well into the 20th century.
Not all ironstone is marked. Very old ironstone made before 1813 and American white "granite ware" may not be marked.
When hunting for pieces, carry a list of manufacturers or a reference book with you. ThePotteries.org has a comprehensive list of British manufacturers available, as well as a great deal of other information. There are also several good books available that list manufacturers, and include photos and registry marks.
Acquaint yourself with the many names that ironstone goes by. They include Chelsea Grape, Chelsea Sprig, Flow Blue, Gaudy Ironstone, Mason's Ironstone, Moss Rose, Staffordshire, graniteware, stoneware, opaqueware, and Tea Leaf Ironstone.
Know that old ironstone comes in many "shapes," as they are referred to. It was--and still is--a workhorse. Along with serving dishes, you may find chamberpots (also called sanitary ware), utilitarian storage jugs, washing pitchers and bowls, soap dishes, snuff boxes and cups without handles.
Learn to spot reproductions, which abound. Look for telltale signs, such as the words "flow blue" stamped on the bottom of the piece. No real piece of flow blue is stamped "flow blue." The words "iron ware" are also a giveaway. You may also see imitation register marks that appear to be stamped on, rather than fired into the finish.