Many things have to be considered when buying antique porcelains. Collecting really rare and valuable pieces needs a little expertise, since many imitations are made. Here are some good advices before paying:
The objects made of original porcelain, e.g. pieces of a table set, are translucent. Put our hand behind the plate while turning it to the light: its shadow can be lightly seen through the material of the plate. For example, classic blue-white Worcester china made in the 1760s has greenish tone to the light. Examine carefully the selected object if there are hair-cracks on it, if the edge is chipped which is a very common failure and lower its value. If the glossy glazing is not even or maybe hazy, it might refer to post-repair. Gently run a needle or blade on the questioned surface. It will run along the immaculate glaze but stuck on the edge of a repair. Knocking it with a hard object or nails is another method. In normal case, it gives clear, a bit deep, clinking voice. If we hear a blunt voice, there is a crack on it somewhere.
It’s not easy to make a difference between the modern copies of antique porcelains from the originals. The lack of glaze on the bottom could be a strong point, where the hand-made pieces were got off the wheel after glazing. There always should be one or two tiny holes on the bottom or elsewhere on a hidden part of the figure porcelains. The gases arose during the burning were off here.
If products of a certain company are collected, always check up the used signs by the firm. These were changed during years but it is always found in special books which sign was used in which time interval. Popular patterns and colors of eras can be strong points for the age of the porcelain. If two marks show different eras, we may hold a copy in our hand. For example, the sign of Meissen porcelain is two swords cross each other, which changed during times but beside this, the number of cut patterns were marked, the number of the maker and the number of the painter also. The English „registered design” had two variations between 1842 and 1883 too.
Always carefully check up the purchasable rarities! There are many books in the topic but a professional can be asked for estimate.