Hi Johnny ; Welcome to Permies!
It sounds like you have the old soft insulated firebricks.
If they are then yes. They can be used as long as their heat rating is high enough.
If your are building a J tube than apx 1800F is a high temp.
If you are have a batchbox expect over 2000F.
I suspect that the heat rating is unknown...
I would give them a try they will probably work fine.
If they do not, then I suggest making a five minute riser with Morgan
superwool
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