There are many varieties of fireclay with different characteristics. If this is "Cedar Heights Goldart", look at what Axner has to say about it:
http://www.axner.com/goldart.aspx
It appears to have a maximum working temperature around cone 12 (2400 F) which is just about suitable for firebox use, but would be vitrified and hard at that temp.
"Clay associated with the coal measures of the Carboniferous System of sedimentary rocks. A few deposits are residual clays similar to china clay. Fireclays are often refractory clays and are used for firebricks. The name gives this impression but there are fireclays which vitrify below 1300°C (2372°F). These are used for drainage pipes, building bricks, sanitary ware and stoneware. Grog is usually fired and ground fireclay."
-The Potter's Dictionary of Materials and Techniques