Captn Roy I am not too sure you are seeing what you think you are seeing. here is not a single pair of trousers/ pants in the photo. the one in the right or the center or any other woman there, what you are seeing there is the tail of their work apron not extending all the way below the hem of their dress or skirt.
As far as welding it would appear they are all welding using oxy/gas in which case they would normally only wear goggles instead of helmets there are some who may be wearing head bands to keep their hair up out of the way.
While arc welding was invented by 1800 it was mainly in the form of carbon arc between to carbon electrodes and adding a filler rod to the molten puddle. Coated metal electrodes came along around 1914 in Sweden. But coated stick electrode welding didn't become popular until years later, tig and mig welding wasn't even invented by 1918.
Any bench welding that would have been done in 1918 would have been done using an oxyacetylene torch. Prior to that it may have been oxygen and a coal gas & hydrogen mix.
I taught a welding class in the Army in the early 70's where I demonstrated how to make a coated electrode in the field for an emergency repair of a broken steering tie rod using a coat hanger or other wire like mechanics wire by making a solution of baking soda salt toilet paper and water. by making a slurry then dipping the wire into it several times then laying it on the hot exhaust manifold of a deuce & a half until dry then used jumper cables connected to the 24 volts of the batteries. After welding it I drove it around the motor pool to show that it actually could work.