As a Bladesmith, I am literally in the heat of things, along with all the other situations knifemakers face in the shop.
For me, NOTHING synthetic, from head to toe. That's more difficult than it sounds. Even during the winter, I have to be careful what I wear. I learned the hard way that I cannot wear a Carhartt vest with the quilted/nylon lining. That HURTS when it melts to you when you get a bit too close to the forge. I love my carhartt vests, but can only use/wear the "sherpa" lined versions in the shops.
Pants..... I personally like Dickies "Workhorse" jeans. They don't break the bank, and are heavy weight, with double thickness/fabric on the front of the legs from thighs to ankles. I thought the Duluth trading "firehose" jeans/pants would be great... but the material contains too much synthetics.... they melt.
For me, I try to wear only natural fabrics....mainly 100% cottons in the shop..... they will burn...but it's a slow smolder...versus a melted blob blistering me with any synthetic materials!
Otherwise,