Is that old tea set porcelain or bone china?
Whether you’re collecting or selling, it’s worth knowing the difference. Not only is it fun to learn something new, the type of material can affect the price of the item.
Disclaimer: While researching this topic, my eyes crossed from all of the geology and chemistry! But since we’re lovers of antiques and not scientists, please forgive me if I simplify this to the absolute basics! (I’ve provided links for those who don’t mind treatises on rock formations and chemical formulas.)
The ceramics family.
The term “ceramic” applies to any number of items, from bricks and tiles to tableware and parts of the space station. All ceramics are basically a combination of clay and heat.
What we care about most are called “whiteware ceramics” — tableware, figurines and other vintage items made from various clays and quartzes.
The basic clays that make up ceramics.
Bone ash is pretty much what it seems: The calcium phosphate ash that comes from burned bones.
Feldspar is a mineral that, when it weathers or decomposes, turns into clay. Its colors range from white to pink, brown, or gray-blue.
Kaolin is a clay found in hot, moist climates such as the village of Kao-ling (get it?) in the Jiangxi province of China.
Bone china and porcelain.
Bone china is the stronger of the two, with a base of bone ash and other ingredients such as feldspar-type rocks and kaolin. It can be fired at a lower temperature (1450 degrees Farenheit) and tends to have a milky, slightly translucent look.
Thomas Frye’s Bow porcelain company was near the Essex slaughterhouses, so he had easy access to bone ash. English potter Josiah Spode took the concept further and helped make bone china popular.
Bone china can appear thin and delicate – yet because its base is bone ash, it’s stronger than it looks.
Porcelain is made from 50 percent kaolin clay, 25 percent feldspar and 25 percent quartz fired at temperatures up to 2500 degrees Farenheit. It was invented in China — and surprise – that’s why it’s called china!
Porcelain doesn’t need to be glazed in order to be waterproof – any glazing is decorative and helps resist stains and dirt. In most cases, older pieces were glazed all over – including the standing rim.
If you see an “antique” with an unglazed standing rim, be suspicious!
Porcelain’s base is white, and even if thick, it’s translucent enough so you can see your hand through it. If it’s decorated, the designs don’t fade over time. Should it break, the pieces resemble glass.
Hard paste porcelain, made from kaolin and petunste (I told you it gets technical!), is primarily used in dinnerware. Soft paste was an early attempt to reproduce Chinese porcelain by adding ground glass, soapstone and lime to the clay.
What does “vitreous” mean?
The term has to do with how much moisture the porcelain will absorb. Because bone china and porcelain are exposed to such high temperatures, their clays fuse together, making them vitreous. Vitreous items absorb less than 3 percent moisture.
Semi-vitreous pieces absorb from 3 to 7 percent moisture, and are often marked as “semi-vitreous” or “SV.”
Why do you need to know all of this, anyway?
For one thing, it’s interesting (hey, I think it is, anyway.)
For another, knowing the different types of porcelain – and the clays they’re made from — could help you date items you want to buy or sell. And that could make it less likely that you’ll be fooled by cheap reproductions or modern fakes.
Up next: Stoneware, Ironstone and Earthenware
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