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In the story I'm writing, my modern-day characters have the need to produce a great quantity of bricks in a relatively primitive setting and society, equivalent to ancient Sumeria. They have access to willow trees, which are a fast-growing species of tree that produces useable firewood relatively quickly, within five years, for a project that is expected to take twenty years or more.

It is recommended to season willow logs for at least 12 months before use as firewood, however, I am considering measures to dry this wood more quickly.

Considering that large quantities of fired clay bricks are required, I am considering something similar to a dragon kiln which can fire a large quantity of ceramics simultaneously.

In order to kiln dry timber, lower temperatures are required than that required to fire ceramics. So, I was considering adding a separate outer layer to the dragon kiln, so that air would be drawn down from beside the chimney of the upper kiln and heated as it travelled down through a series of seasoning kilns beside and sharing a wall with the ceramic kilns. Once the warmed air had passed through the timber kilns, it would then be fed into the burning timber that heats the ceramic kilns, which produce the draft necessary to draw in air at the top of the kiln chain.

Alternatively, cold air in the drying kilns could be heated by the ceramic kilns and pass upwards through the chain of dryink kilns. Either way, the wood drying kilns would operate at around 100°C

Is this a feasible method of kiln drying willow timber for use as firewood, or should I just have my characters put up with year-long or more natural seasoning times?

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  • $\begingroup$ Just make a solar kiln to dry the timber, faster and doesn't require burning half your firewood. the Sumerians could make glass. $\endgroup$
    – John
    Commented Oct 23, 2023 at 21:36
  • $\begingroup$ just out of curiosity is there a reason you want willow and not something far better for industrial use like fast growing fast drying swamp pine? $\endgroup$
    – John
    Commented Oct 23, 2023 at 22:09
  • $\begingroup$ @John It's what's available in a place very similar to ancient Mesopotamia. If you have a better suggestion, I'd love to see it. $\endgroup$
    – Monty Wild
    Commented Oct 23, 2023 at 23:58

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Once the warmed air had passed through the timber kilns, it would then be fed into the burning timber that heats the ceramic kilns

I see no benefit in doing this. The timber kilns operate at basically ambient temperature as far as the ceramic kilns are concerned. You're complicating the problem of delivering and regulating air to the ceramic kiln, and ensuring it'll sometimes (when the drying kilns are freshly loaded with green wood) consist of highly humid air, which if you aren't actively using the ceramic kilns at the same time, may actually condense in them and damage things.

Alternatively, cold air in the drying kilns could be heated by the ceramic kilns

That's a little better, but you're still tightly coupling your production of dry firewood with your production of ceramic bricks. When you need one more than the other, the unneeded kilns and their supplies of raw materials will be physically in the way. If you need firewood more than bricks, heating the empty ceramic kilns will be an inefficient way to heat the drying kilns. Running short on fuel for heating the ceramic kilns will pose a problem if they're your only way to heat the drying kilns producing that fuel.

Additionally, pottery kilns run very, very hot. If there's some kind of accident and the drying kiln overheats, you could be faced with a very large fire and a major loss of fuel.

The needs of the two processes are extremely different, I really don't think it makes sense to combine them.

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I think it woukd be technically possible, but the logistics of the time won't make it feasible to feed the entire process.

You will have your busy time already supplying the kilns with ceramics, drying and storing all that wood could be too much to handle.

Don't forget that once it's dried you want to keep it dry, so it cannot be left to the elements.

You can use your design to pre-heat the naturally dried wood before loading it into the kiln, and use the growth and drying time as a way to buffer the needed resources.

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  • $\begingroup$ You can store the wood outdoors. If you expect rain or snow, you should shelter the top of the stack, but it is important to leave the sides open. A Holz Hausen stack needs no shelter. The dragon furnace is not going to suddenly appear. If the wood needs to season for a year, and the project lasts 20 years, I imagine the firewood store for the next year's firing will build up steadily as the furnace climbs up the hillside. Stacking the firewood around the furnace sounds a bit risky to me. I don't think the time saving of kiln drying justifies it. $\endgroup$ Commented Oct 22, 2023 at 11:39
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You are asking for a technical solution when a simpler solution is available - space or land area. Drying wood simply needs a lot of space which is sometimes covered.

For firewood, you want to split the logs so that they dry faster. Wood dries fastest when the insides are exposed. Around here, it seems that people cut the trees and split them during spring and summer and sell the resulting firewood that fall. It is only when the wood will be used for furniture or other fine uses that it needs to be dried one year for every inch of diameter (or longer).

Also, a pottery kiln will need a lot of wood. A secondary effect of a kiln is the heat radiated to the sides. You can use that radiated heat to help dry the wood.

Stack split logs in multiple stacks all around the kiln with a cover, the greener wood in the outside stacks. You might have a thin wall to help trap heat. Willow is both a softer wood and holds a lot of water. Thus, you need a larger volume of willow to get the heat you need. The drier the better (hotter). If you have the space, cutting, splitting, and stacking one year, bringing it inside for further drying the second year, and using it the third year makes for better firing of the pottery.

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  • $\begingroup$ Around /where/ ? :-) Also as a general point willow is nasty wet stuff and the most effective way of drying is is by air and Sun... one might imagine a very large charcoal stack with a kiln in its centre but then the outside would have to be sealed... $\endgroup$ Commented Oct 23, 2023 at 16:17
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    $\begingroup$ +1 the labor felling and splitting the willow will matter more than drying. $\endgroup$
    – John
    Commented Oct 23, 2023 at 21:42
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You are overthinking it, and you are not restricted to just one wood, you can start with one wood and switch later. We have real examples.

Out of what is available in ancient Mesopotamia date palms is likely your best choice for the long run sustainable choice, this is what they actually used for industrial uses. Willow will work but date palms grow a lot faster. Also remember when you read about where something is good for firewood they are talking about for use in modern homes, industrial firewood is far more forgiving both in moisture content and smoke especially for bricks.

In the short term use use acacia and broom, which are excellent fuel woods known in antiquity. less sustainable but there would be plenty to get you started while other wood air dries. This was the top choice for fuelwood until it got depleted. In real history of the area for smelting acacia and broom were used for decades until they became depleted, then they switched to date palms, 6 months will not even put a dent in your stands.

As for drying you are over thinking it. Split the wood, the smaller the pieces the faster they dry, split firewood dries a lot faster than timber. And they should be using small trees for firewood anyway. they will not be felling large trees for firewood, they don't have easy to use powersaws. A couple of months is fine they are not making cooking fuel for use in a poorly ventilated home they are making industrial fuel. Plus in a Mesopotamian climate drying will be a lot faster than the averages you see when looking up drying times. Those have to average for very wet climates.

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