I was pleased to see the photos of the burls; it's been some time since I had thought of all of the splendid uses Native Americans and others had found for burls, so I hope no-one minds my digression.
Burls are caused by wounds to trees, as has been mentioned. But far from making the tree unattractive, burls had been valued!
They had been sought out by Native Americans as the basis for bowls and drinking vessels often called "noggins." My family had always been delighted to find a good hardwood burl because the grain of the wood in these "wound-scars" was twisted and convoluted. If the usual "wood-grain" resembled layer after layer of straight fibers, a burl could be a hopeless tangle of strands.
Because chisels and adzes are virtually useless in working the wood in burls,
Native Americans would "burn" the burl into the shape they desired.
This has no relationship to burning as in the sense of a wood hearth-fire! Instead, smooth stones would be thrust deeply into a bed of brilliantly-burning embers until virtually red-hot, then moved with green-stick tongs until it was possible to use the stone to begin to burn a pocket in the burl.
Slowly a cavity would be made, with smooth stones being switched out and reapplied as necessary, and the ash and carbonized wood would be chipped away, using stone or metal tools This would depend on the era, and the degree to which the worker wanted to remain true to the traditional ways.
A small drinking cup might take a couple days to "burn out." A larger vessel would take longer, depending on the overall size desired, the depth of he bowl being made, the skill(s) of the artisan(s) and so forth.
I have made various "belt-noggins," with short projections serving the purpose of handles, holes "reamed" therein, and short wood- or antler-toggles attached to a double-strand thong. The toggle could be thrust into a loop of the thong where the thong had been passed behind the belt, allowing it to be carried dangling from the belt. Capacities had varied, but as a rule had been in the 8-ounce to 14-ounce ranges.
Virtually every Native American who had business outside of the community had carried a drinking noggin. Until mining and the keeping of herds of livestock by Colonists led to water-pollution, the majority of "wild water" was relatively clean and safe. (Estimates say at least as safe as most municipal water systems.) People carried noggins so they could dip out a drink without having to crouch or virtually lie down, almost putting their faces into the water as they drank.
People in villages were able to use gourd dippers and cups, and most did so. The fire-carved wooden noggins were heavier-duty and less likely to break miles--and days--from home.
Those I've made were done with smooth, rounded stones a bit smaller than a tennis ball. Prior to use, these stones had been repeatedly fired until I was sure they would not explosively shatter while in use.
I'd heat them until a small stick, touched against the stone, would immediately burst in flames. I'd keep burning the hollow, and outside shape, scraping out the ash and charcoal as necessary (it was necessary when the hollow no longer gushed smoke when the burning-rock was pressed to it) and continuing with this, hour after hour.
So as not to be choked by the smoke, I'd select a site where there was a fairly strong wind always at my back.
It takes me a good two days to burn in the hollow for a noggin, and most of a third day to "stone," or really, to sand, the noggin to shape, removing char-marks.
Between the grain of the burl being so tightly knotted and convoluted, and the fire-hardening effect of burning the noggin to shape, my vessels have never leaked nor cracked, after being packed away or hung on a wall or in a museum case for months or years.
Even the two I'd made of straight-grained wood have stood the test of time and not cracked--so far. Those, I'd sawn to rough shape, then worked down with a reproduction Canadian-made Native American "hook-knife," and a couple Scandinavian "curved knives." Those knives are especially constructed so as to make intricate carving much easier than by relying on chisels.
I will note, however, that after I'd finished, I'd submerged the noggins in molten bees' wax, tempered with nut-oil, for about half an hour, for each one. The noggins had "wept" wax for a little bit, but that eventually stopped.
My noggins are nothing special, but that is because I chose to stop my hand-work once they were suitable for catching a drink.
I must confess, however, that I've often had one with me while I was "playing tourist!" There are public water fountains almost everywhere, but very few have much water pressure. It actually seems deliberate! So, sliding a shallow wooden noggin under the fountain's spout often meant the difference between a decent drink, and either going thirsty or having to buy an over-priced coffee or a bottle of recycled municipal drinking water!
And, I also own some "also-rans;" replica noggins or wooden drinking cups made of close-grained ash or maple, or perhaps hickory. If I'll be traveling and might lose my bag of have it stolen, I'll take something less precious to me.
Although "modern animal husbandry" has long since made swine liver flukes endemic throughout the nation, and various "progressive technological advances" has spread "junk" such as transformers containing PCB-laden oil being lost and various old mining sites having abandoned toxic chemicals where they might get into watersheds, I still carry a good noggin while "out and about."
For one thing, it'as a piece of nostalgia that represents a much less complicated period of time; plus, it lets me take a short breather along the trail in a "much more relaxed" fashion.
Although this is a great departure from the original question about burls, I felt that a digression into the old-time practical uses of harvested burls as something besides a wood-worker's flat "board-slab" harvested exclusively for the ornate grain might be of interest to some readers.
Members of my family had enjoyed the beauty of woods and wood-working, and as a result of this I'd taken up many of the old ways, and I've even combined them with modern techniques, on many occasions.