When Looking at Ceramics, the First Thing to do is Determine if the Item is Pottery or Porcelain.
The easiest way to tell pottery from porcelain is to hold the object up to a strong light source (i.e. 100-watt light bulb), if you see "light" coming through the object then it is made of porcelain if not, it is made of some type of pottery.
There are two basic types of porcelain, soft-paste, and hard-paste. Soft-paste porcelain is oftentimes somewhat "malformed" or misshapen and with the paste having imperfections (i.e. tiny black specks). When held up to the light soft paste will not be quite as thin and translucent (as a hard paste) & will have more of a "pudding" look. The body will be greyish or off-white in colour when compared to white hard-paste porcelain.
Most ceramic items (but not all) have a maker's mark, so always check for a maker's mark. These marks are usually located on the bottom (there are exceptions to this rule, some marks can be found on the back, top or side of an item such as the famous Anchor mark of the Chelsa factory).
Sometimes, you will find no marks or just a series of numbers & other odd marks. When only numbers are found, they usually represent a pattern or shape number, but can also represent the artist who decorated the piece (many ceramic artists were paid by the piece and thus had to identify each piece they painted in order to get paid). These numbers can often time help to identify the manufacturer and date.
To tell the difference between porcelain and pottery (aside from the translucency test) run your finger over the unglazed foot rim or bottom of the item, if it is porcelain it should have a "glassy" feel and be white in colour. A further test is to gently (please!) tap the edge of the item (works best on bowls, cups, plates, etc.) with your fingernail, if it has a clear "bell" like ring it is porcelain...if it "thuds" its pottery.
Most American porcelain found today will date from around the mid 19th century. Early American (18th c.) porcelain is exceedingly rare!
Rarity is one of the most important criteria to look for when collecting ceramics! (after all, there has been a whole lot of it made over the centuries!)
The most collectible period for American Art pottery is from the early 1900s (1900 - 1915).
To determine quality in art pottery, look for an even foot ring, good shape, and glaze (if applicable) and a mark.
Maker's marks that have the words "Made in .....", and or "bone china" are 20th-century ceramics. If the country of origin is listed without the "made in" prefix, (just the word England, Germany or China for instance) then the piece may date from about 1890 to around first quarter of 20th c.
Hand-painted inscriptions alongside a factory mark on porcelain usually indicate a piece of high quality. But, be aware that "printed" inscriptions are found on imitations.
Unmarked porcelain may actually predate the use of marks, therefore, research will be necessary to accurately date it. Be aware that much late 19th and 20th-century porcelain is also unmarked, or had paper labels that have fallen off. You will have to use the item's shape, design, decoration, and body (paste) to help determine origin and age.
Most ceramic marks are printed "under the glaze" (usually a stamp or transfer printed) and appear on the bottom of the piece. Hand-painted marks may be "over or under the glaze", a magnifying glass will help you to determine if the marks are "over" or "under".
You will also find ceramics marks that are incised or impressed into the clay body prior to firing.
On "portrait" plates, always look for the artist's signature on the front of the plate, within the painting.
Marks on early (pre 19th century) English, Continental, and Chinese wares should be viewed with scepticism and not trusted 100%. Many of the more famous marks (i.e. Meissen's crossed swords mark) were copied by other factories. Knowing the details of each factories production will help you determine if it is a genuine piece. A great book on learning period details of the various factories is Miller's, Porcelain Antiques Checklist and Period Detail by Paul Davidson.
Marks on 19th-century wares can usually be trusted and believed.
Early Continental porcelain copied the shapes and decoration of the early Oriental (namely Chinese) porcelains (known as Chinoiserie), later porcelains began to imitate the most popular silver shapes of each era.
An easy method for detecting unseen cracks in porcelain is to balance the item (easiest when testing a plate) on its foot and tap it with your fingernail. If it rings there is no crack...if it thuds, look more carefully as there will be a crack.
To determine if a piece has had restoration, check the glaze for inconsistencies (shiny Vs matte), and colour changes in the paste.
The condition is extremely important in ceramics! Make sure the asking price reflects the condition.
Vases, figurines, and other "decorative" pieces are usually more valuable than utilitarian pieces. The more elaborate and harder to make pieces are also usually more valuable (i.e. large tureens, figural groups, etc.).
There is one crack sometimes found in ceramics that generally does not hurt its value substantially; this is a firing crack (caused at the time of firing). These are acceptable to most collectors and can be recognized by their "random" size and shape. Firing cracks can also be an indication of early production.
Poor quality of design and decoration is a very reliable sign of reproduction or fake. Fakers rarely take the time that craftsmen do when making or decorating a piece of porcelain or pottery.
Always look at the decoration on porcelain under magnification to determine if it is handpainted or printed. Hand-painted wares will show small brush strokes and irregularities in the design.
Crazing is oftentimes (the pattern of tiny "spider web-like" lines) found in glazes on pottery and some early soft paste porcelains, however, they are Not found on hard-paste porcelain.
Although grazing can be a sign of age, it can also be "faked". Natural (time induced) crazing will not have a "pattern" to it, while faked crazing will show a "regular repeat".
Chips in soft-paste porcelain will have a granular, dry texture (very much like flour pastry), while chips in hard-paste porcelain will be smooth and glass-like (another method of determining the difference between the two).
Here is a list of some additional words often found on ceramics; these can help date an item: bone china - after 1915; copyright - after 1858 but usually 20th c.; craze proof - 20th c.; Delft - usually 19th or 20th c.; Déposé - the French word for "registered" 20th c.; detergent proof - 20th c.; dishwasher proof (no-brainer!) 20th c.; handmade - 20th c.; hand-painted - 20th c.; incorporated - 20th c.; limited (Ltd.) - after 1861; East Germany 1949 - 1990; Nippon (found on Japanese wares) 1891-1921; patented - 20th c.; registry marks (diamond-shaped on English wares) - 1842 - 1883. (Ref: Kovels New Dictionary of Marks).