One of the most mysterious things about Danish modern furniture to me when I started making it was this strange soap finish that is talked about so often. I wondered, "What is this all about"?
Well, the easy answer is that it is a soap that is simple washed onto the wood surface. You may wonder how that protects the surface, though. Basically, soaps traditionally where made of oils of some sort or another. Your grandma or great grandma would have used "ivory" soap flakes that where made from vegetable based sources, palm and coconut oils usually. Everything from cloths to who knows what was washed with it.
Why use soap to protect wood surfaces? Wood has pores and those pores will get clogged with dirt and oils from use if not protected. Soap clogs those pore and keeps the dirt and oils out. Also being soap it also releases dirt and oils easily thus keeping it off of the surface.
Wood also benefits by have a finish that slows the exchange of moisture from the wood to the surrounding atmosphere, thus keeping it more stable. Soap finishes aid in doing this like other finishes though to a lesser degree.
The biggest advantage, I believe, is that it leaves woods like white oak and ash looking as natural as the raw wood itself. As a woodworker I love the organic feel this gives a piece. The soap finish is also incredibly smooth to the touch and ages so beautifully.
Some suggest that soap finishes are a lot of maintenance. I have found that since they age nicely I really don't do anything to maintain their appearance. If it does get soiled, simply wiping the surface with a damp cloth will usually remove whatever is unwanted. Wiping the surface with more of the same soap will freshen it up as well. In Denmark this is often used to finish floors even. I think that attests to its durability and effectiveness as a finish.
I purchase my soap flakes from a U.K. based producer. www.msodistributing.com currently can supply this in the US.
Here is how I prepare my soap finish. I mix boiling water and soap flakes in equal parts, 1/1 ratio. If I am making a large batch I will reduce the water a bit. I only want enough water to dissolve the flakes and it should create a whipped cream like consistency when mixed together but with a thicker body to it, not so airy.
First measure out your flakes and then pour the boiling water in equal parts over the flakes. Let that set long enough for the flakes to absorb the water and it will get a translucent appearance and will become sort of jelly like. Now mix in a bowl until you get the consistency I mentioned above. See photo below.
You can now place a small amount inside a lint free white cotton cloth that is folded over itself. Squeeze the soap through the fibers and to the outer surface. This will thoroughly saturate the cloth with soap. Now wipe it on the surface of your wood. Put on enough to fill the pores. Don't let it cake on the surface, though. Once it is dry you can knock down any raised grain with 220-320 grit sand paper. Apply one more coat and that is usually sufficient. Finish by buffing with a soft cloth if desired.
To freshen up your finish follow the same steps. If you have a set in stain you can try pouring some boiling water on the spot. The soap will often force the stain loose. Let it dry and reapply a finish.
A note of caution; Do not wet end grain surfaces to heavily. These areas absorb more water than the face grain and can crack if continually saturated. Thus it is best to make your soap with less water and more like a paste. Also coat both sides of a panel, such as a table top, evenly so that it will reduce the potential to cup or warp.