How to stain wood with ammonia solution

Wood staining with ammonium vapour was discovered by chance in England centuries ago. Oak planks stored in a horse stable have over time turned a dark brown colour. They suspected that the strong-smelling substance from the animals' urine was responsible for the colour change. That's why the craftsmen began to bury the oak planks in dung to colour them. It was later discovered to be ammonium vapour and such a rudimentary treatment was no longer necessary. In the late 19th and early 20th centuries, much of the furniture belonging to Arts and Crafts style was stained by treating the oak with ammonium vapour. Why wood changes colour in the presence of ammonium hydroxide solution, what the advantages and disadvantages of the method are and how it is done in practice, read on.

The reaction between tannin in wood and ammonium vapour is responsible for the colour change

The wood changes colour due to the reaction between tannin and ammonium vapour. The resulting colour is greyish brown with honey tones. Only tannin wood can be stained, which is why the method has been most often applied on oak. Being a chemical reaction, the staining is deeper, not just surface. Basically, the wood changes colour as it comes into contact with the ammonium vapour. The fumes are often popularly called smoke or fumes, which is why oak coloured in this way is also called smoked oak.

Colour intensity depends on amount of wood tannin, which is why it is not a very precise process over which we have total control. European oak stains better than American oak because it is richer in tannin. Western and northern European oak also reacts more intensely than eastern and southern oak. But the amount of tannin can differ from tree to tree, so the recommendation is to use wood from the same trunk when making the object to get the most even colouring.

The colour may be stronger or weaker depending on time of exposure to vapour. The wood begins to change colour visibly 2 hours after exposure and as it is left in the presence of vapour the colour darkens. Being a chemical reaction, it stops when one of the components disappears. The tannin on the surface, which comes into direct contact with the vapour, becomes less and so the reaction decreases in intensity. To obtain an intense colour the exposure time can be as long as 24 hours. High temperature can speed up the process, but it can also slightly change the colour. Smoking at low temperature brings a slight greenish tinge, while high temperature comes with yellow-reddish hues.

Not only oak can be stained with aqueous ammonia solution

Although oak is the most commonly used, any wood containing tannin reacts to ammonia smoking. It can be stained like this gorun, the sky, nuc, mahogany, cherry, chestnut, acacia. The higher the tannin content, the more visible and intense the change in colour of the wood.

Can only be stained with ammonium heartwood, sapwood remains unchanged because it contains no tannin. If the object to be stained also contains albumen, it is recommended that a concentrated black or green tea solution be brushed onto the area before staining. These teas are rich in tannin and can cause the sapwood to change colour in the presence of ammonia.

Why you need to stain wood with ammonia and the risks of the method

It requires a concentrated ammonium solution and an enclosure in which the object can be placed in the presence of vapour. The solution is ammonium hydroxide, i.e. an aqueous solution of ammonia. To be effective, the solution must be of a higher concentration than ordinary cleaning solutions which are somewhere between 3-5%. The ammonium hydroxide used for smoking must have a concentration between 26 and 30%. The solution is unstable and will therefore release ammonium vapour if left free in a glass jar. The fumes are aggressive, attacking the skin, eyes and respiratory tract, so protective equipment must be worn and the process must be carried out in a confined area.

Pregnancy is not difficult to make. Basically you make a wooden frame to cover the object to be coloured. The frame is then covered with plastic sheeting, such as that used in solariums. The idea is that the vapour is emitted in a defined space. A rubbish bin can also be used to cover the object so that the pot of ammonia can be inserted easily at the base. The disadvantage to the dumpster is that you can't see inside at all. It is not recommended to put the object in a plastic bag (a large garbage bag) because the plastic can stick to the wood in some places and those remain unstained.

Before putting the solution in the glass or plastic container (metal and paper soaks are not recommended), use protective equipment: chemical-resistant and higher (like household) gloves, goggles that prevent the vapours from getting into your eyes (swimming goggles are great) and a mask with an ammonia filter. Pour the solution into the pot and immediately place it inside the enclosure at the base on the ground. Use as wide a pot as possible so that evaporation goes smoothly. Check the solution in the pot every few hours and refresh it. What a smoking enclosure looks like, the protective equipment and how coloured the wood is after different exposure times, you can see here.

When it reaches the desired colour, remove the pot from the inside and flush the remaining solution down the toilet or dilute with 3 parts water to be used for surface disinfection or cleaning, if a small amount of soap is added. If you put it back on top of the original solution you risk reducing its concentration. Allow the wooden object to air for a few hours in a ventilated space. The smell will disappear completely. Afterwards it can be protected with any kind of material: oil, wax, water-based or solvent-based varnish, with or without hardener.

Attention! Do not use chlorine compounds in the area and during ammonia smoking. Chlorine and ammonia react, even in the vapour phase, and give rise to products dangerous to health.

Advantages and disadvantages of staining wood with ammonium vapour

The method has, of course, advantages and disadvantages.

Advantage:

• naturally, pleasantly and evenly stains the surface of the wood. The pores are not obvious and the medullary rays do not remain discoloured as with the use of stains,
• staining is not more evident at the fibre ends or on the milled areas, which in berry staining require protection to control absorption,
• the staining is deeper than in the case of staining with berries and so the wood can be sanded without removing the colour (burning of the edges),
• the vapour gets into the most hidden corners so the object is effortlessly coloured evenly,
• wood stained in this way can be covered with any finishing material without restrictions,
• the enclosure once built can be reused every time you want to smoke an object.

Disadvantages:

• is a process that involves certain risks and therefore requires great care and protective equipment,
• you need a "rehearsal" beforehand to know exactly how the process works and what colour you end up with,
• cannot be done in the house or in a busy place. It requires a dedicated space, a workshop or garage, which can be well ventilated afterwards. It is not done in a basement where ventilation is difficult,
• Ammonia doesn't stain repaired areas with putty like the bath does. Those areas will remain unstained,
• if the colour is too dark it can no longer be opened and sanding is not a solution as a lot of wood will have to be removed. The recommendation is to check the staining by putting small pieces of the same wood in the enclosure and gradually removing them. Check the real colour by applying oil or varnish on top. Stop the process as soon as you have reached the desired colour,
• the wood should not be sprayed with the concentrated substance because there will decolorizes.

I hope you find the information useful. As usual, additions are welcome. And if you have any questions or queries, please leave them in the space below. I'm sure I'll reply.