I have great success with the 3/8" tube and getting a good coal is a breeze. I went to OSH hardware the other day and bought 2 feet of 1/2" clothesline rope (label says 100% cotton) since I have some 1/2" brass tubing. When I tried to get a coal it was no use. No matter what I did the coal would go out and not even spread across the end of the rope. I thought "is it the size of the rope? Is there some man made material in the rope hindering ignition? I asked the guys over at Bushcraft USA and one in particular said he had the same problem and believes they put a fire retardant in the rope. That makes sense. Another said maybe synthetic materials but I looked at the burnt end under magnification and there is no sign of molten plastics or other synthetics, also, there was no nasty synthetic smell when I fired it up. Now I did notice one difference and that is the 1/2" rope is very white where as the smaller diameter ropes are grayish. Maybe the grayish ones are not as processed as the white rope? I would try to buy the grayish cast rope if at all possible and be sure the label on the reel or package says 100% cotton. Some cotton clothesline has a nylon core. By the way, the best way to store the tinder tube is in a heavy zip lock bag so its always damp/water proof.