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This tends to protect the use of these marks, and in general restricts them to use on pieces made in the UK.This protects both collectors and the companies who registered the marks.
Once that RHX rate is determined, it is possible to calculate exactly how long ago it was removed from the kiln.
Transferring the method to ceramics has brought additional challenges but initial results have demonstrated that ceramics have the same “internal clock” as bricks. These studies have encountered issues with components within the ceramics causing either addition mass gain or additional mass loss during the RHX measurement process.
The quality of data generated by the Manchester and Edinburgh groups has been due to analysing fired-clay materials which do not contain these components.
The Public Record office and the British Government tend to enforce these marks and registration numbers.
Companies located outside the UK who have reproduced items, and tried to use a facsimile of the marks or numbering system have been sued, and have had sanctions imposed against them.
The method of calculation is based on temperature data for the location, with adjustments for burial depth and long-term temperature variation from historical records.
The ELT is generally close to (but not exactly the same as) the long-term annual mean surface air temperature. Any event involving exposure to extreme heat may reset the "clock" by dehydroxylating the specimen, as though it were just out of the kiln.A small piece of the ceramic is first removed, weighed, and heated to 500 °C, effectively dehydrating it completely.The amount of water lost in the dehydration process (and thus the amount of water gained since the ceramic was created) is measured with a microbalance.There is now strong support for power-law behaviour from analyses of long-term moisture expansion data in brick ceramic, some of which now extends over more than 60 y.Moisture expansion and weight gain are known to be proportional to each other for a specified material at any specified firing temperature.This weight increase provides an accurate measure of the extent of rehydroxylation.