A lot of the times people are puzzled by the usage of these terms and their differences; the fact is they are all types of traditional ceramics. They are all made using the basic material clay. There are other sub categories of ceramics besides these three, so read on.
The differences arise when different clay compositions are used at different firing temperatures and the resultant porosity after firing. Remember raw clay materials if not fired will simply dissolve in water.
Porcelain is similar to bone china but is less translucent since it does not contain bone ash and as a result of which is also less expensive than bone china. It is ideal for use in microwave and conventional oven. The water absorption rate is 0 to 1%.
Earthenware(dolomite) is the cheapest form among all types of ceramic. Since it is fired at low temperatures the water absorption is high and thus is not strong as the other varieties. However beautiful painting can be easily done on them as a result of their absorbing power. It is hand decorated and glazed if to be used for food. Ironstone and dolomite are two quality variations in earthenware.
Stoneware is the strongest of the lot. As it has a higher percentage of china stone than earthenware it is non-porous. It can be easily used in microwaves and is less expensive than bone china and porcelain.
The type of clay used to make a dish, pitcher or plant pot ultimately determines whether the finished piece is called earthenware, stoneware or even porcelain. Just like soil, clay used for pottery has different traits, depending upon its source. Unglazed earthenware stays slightly porous even after it is fired or hardened in a kiln. It is ideal for plant pots as it retains some moisture that helps prevent the soil from drying out. Most earthenware is too porous to go into a dishwasher or even the microwave.
Stoneware, which takes a little more heat to harden in a kiln, is not porous once it's completely hardened. This makes it a good choice for bakeware, dishware and items that will be used or washed frequently. Stoneware dishes are often thick and a bit heavy.
Porcelain takes the highest heat to cure to hardness. Made from a finer clay than stoneware or earthenware, porcelain is durable enough to create even thin pieces such as teacups. Depending upon the glazes used, finished porcelain pieces may be translucent when held up to a light source.
Ironstone Dishware Origins
Ironstone was first developed in the early 1800s in England as a less expensive alternative to porcelain. Experimenting with stoneware clay, early ironstone dinnerware producers crafted a durable and dense opaque material that resembled porcelain and was easier and less expensive to mass produce. Many of the pieces created in the early years of ironstone featured Chinese or Japanese themes, again an attempt to emulate porcelain. Several manufacturers sold ironstone products under different names, such as "English porcelain," "stone china," "new stone" or "semiporcelain," according to Encyclopedia Britannica.
Some ironstone pieces are plain, off-white or slightly bluish, without any designs in the glaze. Designs on such pieces are usually formed into the shape of the item. Some plain ironstone is highly collectible; even Martha Stewart collects such pieces.
Much of the early "flow blue" pottery, initially produced in England for export to the United States, was also made of ironstone. Flow blue items featured slightly blurry designs in blue against a white ironstone background.
Not Always Marked
Not all manufacturers marked their ironstone creations, so it could be hard to tell if a piece is really antique ironstone. Some manufacturers included the word "ironstone" in their makers' marks, which makes such pieces easy to identify. In some cases, collectors' books and organizations such as the White Ironstone China Association can be a great help in narrowing down an era and manufacturer for a specific ironstone piece. Such resources may offer catalogs or photos of makers' marks or of specific ironstone pieces, such as soup ladles or pitchers.