When buying copper pans it is essential to have the correct lining for the cooking task you wish to undertake. For instance, a good plain copper pan should be used for all jam, jelly, chocolate or beer making and a lined pan for all other cooking.
Put simply, if you want the best heat conductivity then go for a tin lining, if you want good longevity in your linings then go for stainless steel or nickel lining.*
Be aware most modern linings are not non-stick but a good old tin lining can be as non-stick as Teflon! New tin linings need to be 'bedded in' use for a period of time on a even low heat - be aware that high heat and dry frying will cause bubbling and tin flow. If you do go for new tin make sure it is even and well spread. Use of sharp or grating implements can cause losses too. It is a good idea to use wooden or copper utensils when cooking with copper.
Tin linings can be very labour intensive and very expensive to maintain if you are not careful - if you are careful they can last a lifetime. (Mine have and very dark too).
A good rule is if you have a pan that should have a good coating of tin and has copper showing through it needs retinning but don't worry too much about any copper that doesn't come into prolonged contact with food - lids and utensils for instance.
When pans are retinned they need to be done by a professional 'Etameur' for good even spread and guaranteed and certified 99% pure tin linings. Amateur tinning can make a good pan difficult to use as the lumpiness interferes with stirring and heat distribution. The tin used needs to be in it's purest form and not in a tin paste, pastes can be used for soldering but not culinary uses! This can be reflected in the prices charged - a good retinning job from a specialist retinner using certified pure tin costs upwards of 2€ or $2.25 per cm so a 20 cm pan will cost between 40€ or $45 to retin, if a retinned pan is wonderfully cheap then it is possibly not as good a deal as you think!
So 'in a nutshell', old tin linings that are in tact and dark are good for cooking, any copper showing on tinned pans needs new tin, new tin needs 'bedding in', new tin lined pans need the 'provenance' from where they were retinned - ask the right questions and you will buy the right pan.
*Silver linings are the most conductive but also the most expensive. Louis XIV of Versailles had solid silver pans for his household!
©Normandy Kitchen Copper 2019