I remember years ago on this forum, another member found clay but his had debris and rocks in it which he cleaned out. It's been so long ago I don't remember how he extracted that out of it. But anyway, yours doesn't have debris in it so that is good.
Definitely you'd need to do tests before knowing how hot to fire. Some clays mature at a lower temp than others. Stoneware, porcelain, earthenware are some of the different clay types and fire at various temps.
The color of the clay doesn't tell all as to what temp to fire to. I've seen a white grey clay be a stoneware clay type and fire to Cone 8. Also, a white grey clay fire to maturity at a lower temp of Cone 06.
What happens if a clay is overfired? The clay will literally melt and can burn a hole in the bottom of the kiln. I'd play around with your clay and make a little pot or fun item and then fire to Cone 018 just to start out with. That's only a china paint firing, a very low temp firing but will give assurance that the clay will fire and won't melt even at the temp for some reason. (My advices are without experience in firing natural clay straight from a dig- keep that in mind!)
Next fire to Cone 06 and how the clay reacts to that. You could then go hotter to Cone 04 or skip that and go up to Cone 2 .
Have fun with it and tell us how it goes! Welcome here to the clay forum!