Lacquer is the heart of traditional kintsugi. It is also expensive. More than 90% of the lacquer used in Japan is imported but there are still a couple of areas which are known for lacquer production, Iwate prefecture is one of them. Below is a short summary of how lacquer is gathered, taken from a couple of PDFs, one from Iwate prefecture, a major area of production.
A lacquer tapper generally works up to 500 trees per year and although there are many variables such as the skill of the tapper, size and quality of the trees, the goal is to harvest about 75 kg of raw sap. The selection of high lacquer bearing trees is crucial, mature tree will yield about 200 ml of lacquer so a total of about 100 liters of raw lacquer. The amount of lacquer locally produced in Japan has fallen more than 95% since the 1960s.
The trees are worked on a 4 day rotating cycle from the beginning of the rainy season, around June, to the end of October. A tapper will select four sets of trees to tap, around 100 – 125 trees in a set for a total of 400-500 trees, going to a set a day and then not returning to that set until the other 3 sets have been tapped which leaves the trees 4 days to heal from the scoring cut. To start a tapping area an incision of about 2 cm. long and deep enough to get through the bark but not so deep as to scar the living wood is made just above the ground on one side of the tree and then again on the opposite of the tree about 20 cm higher than the first cut. This pattern is repeated so you will have an initial cut on either side of the tree spaced roughly 20 cm vertically. This initial cut, called medate, serves as both a visual que for the tapper on where to make the next cuts, and a signal to the lacquer tree to increase lacquer production to help repair the cuts. The cut in the tree triggers a healing reaction which is why the lacquer flows, to heal the cut. After the initial cuts are made on all the trees in the set the work of the actual tapping starts. Cuts are made above the initial cuts which are slightly longer, and the secreted sap is scrapped out and collected. A days work will consist of going to every tree in that days set and on every tree doing a cut above the initial cut. The pattern left on the tree after a series of 18-20 cuts, will be a series of V shapes on both sides of the tree, staggered from one side to the other, up to below the branch areas. Too many tapping areas or ones not placed well can result in the tree failing and being damaged. The lacquer from the initial cut to about the 7th cut is called beginning lacquer with the prime lacquer coming from about the 8th to 18th or so horizontal line above the initial 2 cm cut. The season for this beginning through to prime tapping is about from June to late September. After this there are a couple of more tapping methods which produce lower quality lacquer with the final cut being around the entire circumference of the tree, this last cut stops the flowing of the lacquer. The lacquer grade is gauged from when in the harvesting season it is gathered.
https://www.youtube.com/watch?v=adjHeXAir2o very good visuals
https://www.youtube.com/watch?v=5pVc5zvdVqI English subtitles
https://www.youtube.com/watch?v=1auomY06Lf0 beginner tapper from Iwate prefecture
The #1 figure shows the chemical makeup of lacquer, #2 shows lacquer taken from the first 7 tapping cuts, #3 shows the prime lacquer taken from the 8th through to the 19th or so tapping cut, #4 is the last cuts for tapping.
Image from https://www.ffpri.go.jp/pubs/chukiseika/documents/3rd-chukiseika3.pdf
The left image is a schematic of the tapping sections, the right top shows a compete set of cuts from the initial cut up through to the last cut, and right bottom is a sideview of the depth and spacing between the cuts.
Image from https://www.pref.tokushima.lg.jp/file/attachment/436106.pdf
These images show both the cut for the tapping, the size of the trees, and an image of what the tapped trees look like.
Image from https://www.pref.tottori.lg.jp/secure/1326827/0718urusi.pdf