We’ve planned to make two type of tests with different sort of wood samples.
- Darkening different oak samples with ammonium – fuming
- Crack test with the non-tropical and tropical wood samples from the leonardo guitar research project (this test is cancelled and rearranged as a fuming test with tannin acid ,because of the diversity of the samples according to the grain direction and sizes cannot make any conclusion and the results will be not significant)
The Fuming experiment
“Ammonia fuming is a wood finishing process that darkens wood and brings out the grain pattern. Unlike most other finishing techniques, the fuming process is colouring the wood all the way through instead of only superficially. It consists of exposing the wood to fumes from a strong aqueous solution ammonium hydroxide which reacts with the tannins in the wood. The process works best on white oak because of the high tannin content of this wood. Fumed oak is also called smoked oak. Other species may also be fumed but usually will not darken as much as white oak.
The wood to be fumed is placed in a sealed chamber with all the surfaces to be fumed exposed to freely circulating air. A large shallow container of ammonium hydroxide solution is placed on the floor of the chamber and the chamber is sealed. If the chamber is large or the fuming is to be done for a long time then more than one container may be provided or the ammonia may be replenished during the process. The fuming time depends on the amount of darkening required, the size of the chamber, and the strength of the ammonia used. It is usual to oil the wood after fuming to fully bring out the effect.
Fuming has an advantage over staining in that it does not obscure the grain, it just darkens it. Unlike staining, there is no possibility of blotches or runs. Fuming is also colorfast. Fuming has the disadvantage that it is not a very precise process. Different batches of wood will react to fuming differently. For this reason wood that is to be fumed for a particular project is often taken from the same tree. Even so, boards from the same tree, and even different regions of the same board, can have a noticeably different color. Where a consistent color is important, staining or dyeing may be better options.
Fuming has some inconvenient safety issues. The solution of ammonium hydroxide used is much stronger (26% to 30%) than in household ammonia and is corrosive. The fuming must be done in an enclosed sealed chamber. Ammonia splashes can burn skin and the fumes can cause burns to eyes and lungs. Operators need to wear gas masks, gloves and eye protection.
The darkening of the color relies on the ammonia reacting with tannins in the wood. The process is most usually applied to white oak as this wood has a high tannin content. Red oak may turn greenish rather than deep brown. Other species may not darken so noticeably as white oak, or at all, depending on the tannin content. The effect of fuming can be enhanced in non-tannic woods by applying a coat of tannic acid to the surface before fuming.”
We had 5 types of oak samples:
- Already smoked oak (S)
- Holm oak – Quercus ilex (E)
- oak from floor (F)
- oak from table (T)
- Local oak (L)
All these samples were prepared and numbered before by Amélie. For the relatively small plastic container we created a 3 level compartment to hold above the samples from the ammonium. After the box have been loaded with the wood pieces, we filled the bottom of the container with the 25% ammonium hydroxide and closed it with the lid, and an extra plastic bag added around to insure air tightness.
After 1, 6, 19, 44, 68, 92 hours we took out one sample from each type of the oaks and compared them with the control samples. We only took out samples when we noticed a change of colour, which explains the irregular time intervals of take outs.
The other test we had is with the wood samples from the leonardo guitars. After they have been cutted and marked, we made 3 groups:
- control samples without any treatment
- samples with fuming
- samples with tannin acid and fuming
We used the following mixture to submerge the samples of 3. for 22 hours in tannin acid and let them dry an extra 22 hours before fuming :
- 400ml 96% alcohol
- 4 tablespoon of tannin acid powder
During the tannin acid treatment we shaked the compound regularly for the optimal effect.
Both tests are still running, results and conclusions will come later!