Originally Posted by Carlo Bartolini
Grat info Chemmy - saved to my files, is the tannic acid in very strong "undrinkable" black tea enough for that? (it does contain some oxalic acid)
My dear Friend Carlo, nice to hear from you again. Actually Black Chinese tea [in it's original blocked form as shipped from China, has the strongest amount of tannin on the market. [easily found in china town, not sure about on the high seas lol.] and if you add a pinch of sodium hydroxide to it instead of oxalic, it will help the penetration even more so. but for the best affects, other chemicals will be needed for color or dimensional changing.
"Pure" tannic acid, or pyrogallic acid from the gall nut of oaks, are the 2 best sources for it though. It is a very strong liquor and has been used in wood finishing in formulas to treat woods such as and mainly on figured maple, but will work to readily to prepare any wood that will be receiving other chemical coloring upon it such as ferrous sulfate, in this instance. . It gained great strides by the wood dyers in England when they produced the still popular "Hare-wood" Finish. One that was a light to medium grey, struck fully through the dyed veneers of curly Sycamore.
It was found later that a saturated solution of TA followed by very weak solution of the FS in hot water [120-140 degrees] could bring out the figure in maple in a most excellent way, just as Potassium dichromate does with figured mahogany, in it's own way.
The Real "trick" is to not get it concentrated enough to turn the wood grey as it would for the hare-wood finish color. At the very most you would want it to be ever so slightly grey, no more than the natural occurring grey or less, in this case, as the burl itself has, which in some areas is pretty grey already. And less in other figures. It's a fine line of mixing and testing, that's why i always start out weak so as to not have to keep adjusting much as my trials continue. Always easier to add, than to take away lol.
Oxalic acid, my friend, is only really used in our trade as chelating agent, it has the ability to grab on and hold metal oxides as iron and thus is used to remove any type of iron stains including old iron-gall inks etc. It also has a light bleaching action which may be useful, but seldom is really necessary, as the surface, once stripped and treated and sanded to any degree, removes the thin layer of this.
Never use in shellac as you already know my friend.
Chemmy / sam