RuneScape Classic Wiki
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RuneScape Classic Wiki
The main RuneScape Wiki has an article on: Steel bar certificate

A steel bar certificate is a certificate that is obtained by exchanging five steel bars with Miles at Draynor Village market or with Watto at Zanaris. Steel bar certificates are stackable and each one can be exchanged back for five steel bars by talking to Miles or Watto.

Because steel bars are not stackable and players can only trade twelve items per trade screen, they are often exchanged for certificates so that they can be traded easier and faster.