Why the marks are important:

 The object of a ceramic trade mark was to enable at least the retailer to know the name of the manufacturer of the object, so that re-orders, etc., can be correctly addressed.

 In the case of the larger firms the mark also has publicity value and shows the buyer that the object was made by a long-established firm with a reputation to uphold; such clear name- marks as Minton, Wedgwood, Royal Crown Derby and Royal Worcester are typical examples.

 To the collector the mark has greater importance, for not only can he trace the manufacturer of any marked object, but he can also ascertain the approximate date of manufacture and in several cases the exact year of production, particularly in the case of 19th and 20th century wares from the leading firms which employed private dating systems. With the increasing use of ceramic marks in the 19th century, a large proportion of English pottery and porcelain can be accurately identified and often dated.

How marks are applied:

Ceramic marks are applied in four basic ways:

(a) Incised

(b) Impressed

(c) Painted

(d) Printed

(a) Incised into the still soft clay during manufacture, in which case the mark will show a slight ploughed-up effect and have a free spontaneous appearance.
This type of mark is usually used by small volume studio potteries. 

(b) Impressed into the soft clay during manufacture, many name-marks such as 'Wedgwood' are produced in this way from metal or clay stamps or seals. These have a neat mechanical appearance.

(c) Painted marks, usually name or initial marks, added over the glaze at the time of ornamentation, as were some stencilled marks.

(d) Printed marks transferred from engraved copper plates at the time of decoration. Most 19th-century marks are printed, often in blue under the glaze when the main design is also in underglaze blue.

Information on the method of applying each mark can be of vital importance, for instance the early Chelsea triangle mark must be incised not impressed, as it can be on 19th-century fakes.

General Rules for dating marks:

There are several general rules for dating ceramic marks, attention to which will avoid several common errors:

 
(1) Royal Arms: - Printed marks incorporating the Royal Arms are generally of 19th or 20th century date.

(2) Pattern Name:- Printed marks incorporating the name of the pattern are after 1810.

(3) 'Limited' Company Marks incorporating the word 'Limited', or the abbreviations 'Ltd', 'Ld', etc., denote a date after 1861, and most examples are much later.

(4) Trade Mark:- Incorporation of the words 'Trade Mark' in a mark denotes a date subsequent to the Act of 1862.

(5) Royal:- Inclusion of the word 'Royal' in a firm's title or trade name suggests a date in the second half of the 19th century, if not a 20th-century dating.

(6) Registered number:- Inclusion of the abbreviation R N' (for Registered Number) followed by numerals denotes a date after 1883 (see Registration numbers).

(7) 'England':- Inclusion of the word 'England' in marks denotes a date after 1891, although some manufacturers (Thomas Elsmore & Sons for example) added the word slightly before this date. 'Made in England' denotes a 20th-century date.

It was William McKinley (the 25th president of the USA) who introduced the highly protectionist McKinley Tariff Act of 1890 - this imposed tariffs on many imports (including pottery) in order to make it easier for the American manufacturers to sell their products. It was a requirement of this Act that all such imports carried the name of the country of manufacture. 
This provided well-known marks such as "Bavaria," "England," "Nippon," - indicating the country of manufacture.
In 1921 the Act was amended to require the phrase "Made in" preceding the country of origin, The labeling at individual British potteries varies somewhat from the 1891/1921 dating requirements described above (e.g., Wedgwood adopted the "Made in England" around 1908/10 and may have used it on some pieces as early as 1898),

Mark of Thomas Elsmore & Son (1872-87) with the use of 'ENGLAND' before the mandatory use after 1891

(8) Bone China:- Use of the words 'Bone China', 'English Bone China', etc., denotes a 20th-century date.

(9) Descriptions:- Use of words of description such as "Ye Olde Willow" "Genuine Staffordshire Ware" "Victoria Ironstone" and the like usually indicate modern copies.