Hello, and i have a question about clay lining a forge

First off I would like to say hello to the community here and I look forward to receiving (and hopefully giving some day) great advice.
So on to my first question. I just recently acquired an antique champion forge at an auction. I am going back and forth on the decision to line it with clay, or not. A coworker who has been doing some smithing for years told me that, he has seen both lined and unlined, and that as long as you keep the fire controlled, there is no real need to line with clay. However, another group I belong to on FB, the overwhelming majority said, line it.
Second question. What type of clay do I use? I have heard everything from refractory cement to artist clay to clay you dig up in your back yard.

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