My research into disodium octaborate tetrahydrate (DOT), aka borates
Summary: Don't
First, I'm looking at the materials mentioned in the thread and the underlying problem for all is that they do not penetrate throughout the deadwood. There will always be the likelihood that the inner deadwood is rotting away, and one day, the surface of the deadwood will collapse like a deflated balloon.
Then I started looking into what would penetrate and kill the problem, even if it doesn't harden punky wood. I could kill all the fungus with a fungicide to protect the wood from rotting, then cap the deadwood with a hardener. That search lead me to "borates" which lead me to
THIS RATHER LONG MASTER'S THESIS (thank you, Aliya Ann Turner for doing the work, I hope you got your degree) where I got the full name disodium octaborate tetrahydrate (DOT). I had used a product containing this compound to protect exposed studs while rebuilding my home after Hurricane Katrina. "Bora-Care" boasts that it will penetrate far into the stud (or all the way through) taking DOT with it. Looking to see if borates are toxic to plants, I found several articles that reminded me that I had missed an important element in the first word of DOT: diSODIUM
Borates are salt acids.
Turner's paper (link above) shows and explains how borates penetrate the sapwood portions of dried lumber. Borates can even be leached from treated wood to form DOT crystals on the surface of the wood. All well and good for preserving historical woodwork. But what about where deadwood meets the living? Turns out, it's bad news. As a salt, borates function as a mover of water. Ever burn a section of grass, or foliage of a bonsai, by leaving too much fertilizer in one place? Many fertilizers are chemical salts. When high concentrations, even dry crystals, rest against leaves, water is drawn from the cells faster than the plant can replenish it. Then the affected cells die and turn brown and we say we "burned" it.
I came here to look for mentions of borates and I found this 2011 post by
@crust:
I tried borates on some old rotten trunks and it was clear after two years this treatment leaches out and kills the tree outright-kaboom. Don't use borates.
Note his use of the word "leaches". Imagine, then, burning the living tissue of your plant from the inside. Borates draw water from the living tissue. The living tissue is being replenished by the roots, but the borates always want more. The water is drawn out further and further by the salty deadwood. Away from the xylem and out to the surface of the deadwood where the sun and air take it away. Water that is supposed to be flowing to higher points in the bonsai is being redirected into dry dead wood. This is done, not as a crystal sitting on a leaf, but an area as big as where the deadwood you desire is in contact with living tissues.
Could an equilibrium be found where there is just enough borates in the deadwood, but they do not leach water from living tissue? No. It's a salt. It will always be in the tree sucking water away.
And those DOT crystals wicked out on to the surface of the deadwood? It gets washed into the soil, of course, where the borates find a new friend: your roots.
@crust is right. Borates leach the life out of trees. I need to pay attention to him more.