Line a big stainless pot with an old t-shirt, fill it with wood ashes, then hot water, and let it soak for 20 minutes. Then I bundle up the t-shirt and squeeze out the liquor. That's mild potash solution. Use rubber gloves. Alternatively, buy potassium carbonate on eBay. It's the pure version of wood ashes. I've gone from wood ashes to the powdered stuff just because it is less work. Don't use regular lye because it attracts moisture.
Then put in a coil of the hemp and simmer it for a couple of hours. The potash (potassium carbonate) cooks the lignin out of the rope. It weakens it but reduces the ash formation. This was called "bucking" back in the day. The water will be brown. You might want to buck the rope twice to really get it lignin free. The rope will go from tan and rough to whitish and soft.
Then wring out the rope and soak it in dilute vinegar to neutralize the potash. Then rinse and dry.
You can use it like this or soak it in a dilute solution of potassium nitrate (saltpeter). The bucking is really the most important part.
I use the KNO3, but I have to be restrained with it because too much will make the match spit sparks, which is unsafe. If you use the KNO3, dry the match flat on a cookie sheet. Otherwise the KNO3 will migrate to the low points and make the burn rate uneven.
Twice bucked match with KNO3 is sure fire.
The sign of success is match that will burn with little ash and a big ice cream cone shaped glowing end. If it has a round nub of an end, try something different. It takes experimentation.
