I thought this was the way to make a gradual transparency. Glass shader followed by a surface shader masked by a greyscale texture, as texture on card. I added a firetexture (color) to the luminosity input.
But I get a strange background, so this might not work at all. It was even worse when adding a second card for smoke, done the same way. I tried several variables in the glass shader, but to no avail.
Any feedback is greatly appreciated, as always.
My aim was to add flames to the logs. Clouds are too 'rough' for these tiny sizes.