Acid Condensation Issues with Drying Kilns

This discussion of drying Oak for iron-and-wood furniture purposes takes an interesting detour into a discussion of acidic conditions in wood and kilns. October 15, 2010

I was wondering if anyone has any input on the Sauno wood kilns? It looks like a nice way to do small scale kiln drying. I have access to a lot of highly figured wood for reasonable dollars and will mill it myself. I have a Logosol M7 and a slabber. The picture below is an example of my work. The top is a 4'X8' chunk of black walnut.

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Forum Responses
(Sawing and Drying Forum)
From contributor G:
I note from its manual: "Please note that oak must be seasoned outdoors at least half a year in order to get rid of the corrosive acids that could destroy the aggregate." It seems to be little more than a steam generator used mostly as a heat source and occasionally (?) as a stress relief steamer.

Using your beautiful table as an example, I note that black walnut sapwood can be evened out by steaming. The cost is a total loss of all those wonderful highlights in walnut - the subtle variations in the redís and other colors that you see in the wood. Steaming, while it does darken the sapwood, renders the whole wood a nasty evenly bland brown. I think you would be happier with a dehumidified kiln which, on the other hand, does not damage the color and generally works at a lower temperature so that the stress the steam could relieve doesn't happen in the first place.

The unit seems underpowered given the very slow heating rates given. That may be untrue or pointless for you however. I also note that the site you sent us to suggests that it can properly dry 100mm lumber in five weeks - perhaps some softwoods, but that seems unlikely in oak, cherry and walnut. Perhaps I miss the point somewhere and someone who actually has one will speak for it.

From contributor Y:
The moisture removed from oak is high in tannic acid - higher than most other species. Its ph level is high which makes it corrosive. I am sure this is the basis for the entry in the manual Contributor O points out. By seasoning outdoors for half a year most of the moisture thus corrosive acids are indeed removed. I agree with Contributor G that a small dehumidification kiln might be a good fit.

From the original questioner:
Thanks for the replies (and compliments). I didn't catch the part about six month air drying for oak, which is a good part of the wood I will be cutting. I am looking to save money on wood for tables and such so I thought that the Sauno might be a good idea. The table top was over $3,000 when I bought it. The economy has been pretty devastating on my business so I am looking for ways to make my work more affordable. I have primarily been doing architectural ironwork - fancy railings, driveway gates, fireplace doors and screens which have died right out so I am looking at doing more tables and furniture. I have started looking at some of the solar kiln stuff on this site, and will be looking at the dehumidification kiln information also.

From the original questioner:
The point of what I am doing is not to lower prices, but to keep more profit in my pocket. If I can sell some excess lumber that's even better.

From Gene Wengert, forum technical advisor:
It is interesting to read the claim that air seasoning for six months will remove the acidic acid. I wonder where it goes? What about the huge amount of checking and warping from air drying that long?

As a special note, concrete in a kiln should use river rock and not limestone when drying lumber to avoid deterioration of the limestone. Of course, insulation of the floor, roof and walls of any kiln is a good practice to save energy. It seems that the Sauno kiln is indeed well insulated.

Generally we would not think that oak thicker than 5 mm can be dried in six months, so the idea of 10 mm oak being air dried in six months is questionable. In fact, 12/4 is nearly impossible to dry conventionally. Even 4/4 is questionable if stacking in October in the north; it would not be very dry by April 1. I also agree that 10 mm of most species cannot be dried in the short time indicated.

Note that oak is seldom dried over 130 F (45 C). I think that the literature talks about 75 C. That seems too hot for oak and many white colored woods. Using this equipment for American hardwoods may take some development; the picture is a softwood, which can take this heat and would not require close control of drying conditions for good quality. One of the big problems in our industry is that the consumer of furniture and cabinets and flooring is buying from overseas. Too bad they didn't but American, as our industry and lumber prices would be so much stronger. Does the same concept apply to buying non-American equipment versus American?

From the original questioner:
Hi Gene, thanks for your input. I was actually hoping that you would chime in. I have ordered the book you recommended on drying wood to start the learning process. As for the tool comment (a very pertinent question), I usually buy as American as I can. I have a Big Blu air hammer for example made by a small manufacturer. One real problem with "American" made tools is very few are actually produced in the states now.

From contributor Y:
Gene, over twenty years ago when I started drying with my dh kilns the manufacturer, while graciously answering many questions for me, informed me about the acidic acid thing you seem to disagree with. Since I knew very little about it and they intelligently answered every single question and solved every problem I had I believed what I was told. Is the acid thing not true? What then deteriorates the limestone? I have not experienced any deterioration in my limestone concrete and my kilns run 24-7. I do not claim to know it all or even much for that matter. I am just interested in the subject.

From Gene Wengert, forum technical advisor:
The vapor that comes out of oak is acidic. So, to avoid damage, you can use resistant materials or you can avoid condensation. Many species have very little acidic vapor moisture. The important issue is that the wood remains acidic even though it is partly air dried.

From contributor Y:
Does it not stand to reason then, that the more moisture that is removed through air or shed drying the less acidic vapor produced afterwards in the kiln?

From contributor C:
I have seen the discharge from oak in vacuum kilns being much lower than six. I have seen a lot of damaged concrete in oak kilns. So does the dissolved tannic acid end up on the floor with the capability of dissolving limestone? Doesn't the dry distillation of wood yield acetic acid?

From Gene Wengert, forum technical advisor:
PH of 7 is neutral. Lower than pH 7 is acidic and higher is basic (or caustic). Typically wood is around 4 pH. We need to talk about acetic acid primarily and not tannic acid.

Acetic acid vaporizes at warmer temperatures, but not all is removed when drying, especially air drying. Tannic acid does not vaporize as easily, so very little is found in the air or in condensed air when drying. Although the acetic acid does vaporize, when it hits something a bit cooler than the wood, it will condense. In fact, it is not unusual to see water condensation in a kiln too. Due to acetic acid and any other trace amounts of other acids, this water condensation will be low in pH; 3.5 is found often. Condensation is more likely when operating at cool temperatures and high relative humidityís, and it is cold outside. Very little damage will occur at high temperatures with low humidityís as the surfaces in the kiln are not cold enough to permit condensation. On the other hand, there can be spots cold enough within a wall; hence we try and seal the interior of the kiln walls so that vapor cannot move into a wall and hit a cold spot. In other words, condensation can occur on the floor surface or in the interior of a wall. If in the interior of the wall, the condensed liquid is likely to stay there for a long time and do a lot of damage. We do suggest that a floor be insulated so that it will not be as cold and therefore less likely to have condensation. We have noted that bacterially infected wood is more acidic than uninfected and that the acids also seem more corrosive. Acetic acid does indeed react with lime; in fact, all acids will. In fact, even aluminum will corrode when the pH is low enough. Acetic acid is found in oak.

When steaming into a kiln, the steam itself is 100% water. However, the boiler water needs to be high corrosive (pH = 11 or so). If the boiler ever burps and high pH boiler water gets into pipes and even into a kiln, damage to the structure can be severe. For this reason, we use traps and other design features to avoid this happening, but if the boiler pressure drops excessively (due to lugging the boiler or overloading it), the risk of high pH water into the pipes and kiln is high. The risk is high with small boilers; large boilers have lots of prevention techniques.