In 1914, the standard HE used by UK artillery was Lyddite, a formulation based on picric acid, this was a powerful explosive but expensive. TNT was introduced, but this too was expensive, particularly in its pure form required for shells so eventually Amatol was adopted. This was a mixture of ammonium nitrate and lower quality TNT, various proportions were used but eventually 80% ammonium nitrate and 20% TNT became standard. The parallel inside walls of the shell made it suitable for filling using pre-formed blocks of explosive as well as pouring.