Where easel paintings are on display in a hostile environment, such as where there is no total air conditioning system, it is advisable to protect them by glazing the front and covering the back with a board. This is also necessary when pictures are sent on loan, as the recipient's environment may not be to the same standard as the original display or storage areas. In most cases, this involves adding a modification to the back of the frame, fitting glass or acrylic, spacing slips, assembling and screwing on a hardboard back. In Norwich during 1970-1971, a method was developed incorporating a sliding back which reduces the final assembly time and gives quick access to the picture for conservation or photography.