If 19th century workers needed to be inside an active volcano for several hours, would they have materials to make a proximity suit to do this? Assuming working temperatures of 900°F for 4 hours, and the time is 1890. Assume they have chemical oxygen candles (rebreathers) as well.
Iron workers were probably the closest human workers have ever gotten to working in Weyland's smithy.
Do take note that in the 19th century, the fashion for iron workers appears to be britches, vests and bowler hats; or else top hats and frock coats.
Asbestos has been known for ever, and according to this resource, seems to have been known and used for fireproofing clothing for a very long time. I think it comes down to a matter of timing: the industrial mining, use & application of asbestos just came too late for 19th century volcanic spelunkers.
In a world where the properties of asbestos are known, where it's relatively easy to obtain, and where the need to work inside volcanoes has spurred the growth of this particular industry, I see no reason that such a suit couldn't be made, if crudely, in the 19th century.
You will need to remove heat from the workers.
900 F is too hot to breathe. The workers will need diving suit - like helmets to provide cooler air. Since they are in suits they can also have a water circuit to cool the suits. The hose providing cool water will itself stay cool from the water inside it and the water hose can jacket the air supply hose which otherwise would itself catch fire.
The materials for the suits are not waterproof; the opposite as much of the water pumped into the suit comes thru the canvas pores and drips away or evaporates, carrying away heat.
You would need to keep the cool water coming fast to keep your workers cool. You had better have a lot of it topside if they are going to be in 900F for 4 hours. You could pump the water back up to reclaim and cool it. Alternatively it could just fall out the pores of the suit and then it would keep working to cool the immediate environs of the workers.
There would be a lot of steam!