Sheet for x-ray tubes.
An example of the element Tungsten

"Pure" tungsten sheet, a popular seller on ebay, these are from a major SNAFU at a factory where they were making x-ray tube targets. In a nutshell x-ray tube tungsten must be processed by vacuum sintering and hot rolled into an essentially solid sheet for reasonable life, this material was hydrogen sintered and in as pressed condition, OOPS! Anyway to make a long story short they didn't know until they tried the burn-in and almost all the tubes failed. Many, MANY thousands of dollars of tungsten, work, and anode assemblies were wasted all because someone decided to buy the WRONG material because it was cheaper.