Do bees put holes in the propolis layer on purpose to achieve the effect of functional clothing?
Torben Schiffer says so. He presents in the name of HOBOS electron microscopic images of holes in the propolis layer online among a theory of their formation and effect. It is stated that bees create these holes on purpose to ensure the removal of moisture by water vapor diffusion. Supposedly, they achieve an effect such as that of functional clothing (like Gore-Tex) by this means that allows water vapor diffusion but prevents the permeability to water in its liquid form. Several questions arise from the article.
Different amounts of propolis on different surfaces
The article begins by stating that bees cover end-grain wood (grain is directed toward the room) with propolis extra consciously. But is that really the case? The images that are presented show unplaned blocks of end-grain wood, the surface has not been smoothed. Do bees put more propolis on unplaned end-grain wood than on unplaned wood with grain that runs parallel to the wall of the hive? Do they cover the floor and ceiling of a tree hive more consciously with propolis than the walls? My observation is that the increased cost of material solely results from the roughness of the substratum.
Interaction between wood moisture and relative humidity
The description of the processes in which water vapor is absorbed by the wood is wrong. Water vapor diffusion/sorption does not necessarily result in condensation of the water. The wood largely binds the water vapor chemically and physically by means of sorption. The water vapor is not available in the form of free water e.g. for capillary transport. The absorbed water cannot “flow back” into the hive. When the temperature drops, condensation of the water vapor can certainly occur. The water would then be distributed and absorbed by the wood. Theoretically, the water could get back into the tree hive subsequently. But, tree hives have extraordinary thermal insulation on the bottom and at the top, comparable to passive houses. A drop in temperature and subsequent condensation can only take place deeply inside the wood and do not pose a threat to bees in my opinion.
The consequences of fiber saturation of the wood for the tree hive ceiling
The explanation of the term “fiber saturation” is wrong. Fiber saturation does not mean that all pores and channels are filled with liquid water. It is the maximum wood moisture that is achieved by means of sorption when the relative humidity is at 100%. There is (almost) no free water in case of fiber saturation yet as it is largely bound chemically and physically. As well as the explanation of the term, the conclusion that sorption (or water vapor diffusion respectively) can cause 90% of the surface of the tree hive ceiling to be made up of water is wrong.
Surface diffusion and its part in water transportation
The following statement from the article is greatly exaggerated: “Even at partial fiber saturation a part of the water would get back into the interior against the water vapor pressure because the relative humidity is usually lower there.”
This refers to the surface diffusion, it describes movements of wafer-thin film of sorbed water on the pore walls. In some cases, water vapor diffusion also causes water transportation in opposite directions, which does not depend on the temperature, but exclusively on the difference between the levels of relative humidity. The amounts of water are minute and practically irrelevant in building physics. Especially in the case of the tree hive I consider them to be marginal and negligible because of the high wall thickness.
Torben Schiffer compares the effect of the propolis layer with functional clothing
A core assumption of the article is that the propolis layer applied by bees retains liquid water but allows water vapor to permeate. It is explained with the holes in the propolis. The article only delivers evidence for the presence of the holes by means of electron microscopic images. Experiments/results on water vapor permeability are not presented.
The samples that were photographed and examined originate from a hive that was newly prepared at the time and in which a bee colony lived only for the summer of 2016 before it perished. While it may have appeared like the bees fully covered the lid with propolis around the breeding area, about half of the lid was not coated and especially the transitional areas were quite patchy. It could be argued that the holes that were found rather appeared because the colony perished prematurely and did not finish the coating. Were holes found in other samples from other hives, maybe even in tree hives?
The electron microscopic images suggest that the holes found in the propolis are minute. They are not. As the keys of the respective images show, they have a diameter > 100μm (i.e. 0.1mm). Holes of this size can be detected with a simple stereomicroscope or magnifying glass. Therefore, everyone can investigate the theory that was presented using his or her own hive. I made an effort to verify it using an older hive that had been in use for many years. I did not find a single spot on its lid that was not covered with propolis.
The following image shows a sample from the lid of a hive that Torben Schiffer’s samples also originate from.
The comparison with breathable clothing is absurd. The holes in the propolis that are shown in the pictures have a diameter > 100 micrometers (0.1mm). The openings do not hold back any liquid water, they can already partially be categorized as macro-pores. To recreate the effect of breathable clothing the openings would have to have a diameter that is at least 1000 times smaller.
So instead of 1.3 billion micro-pores per square centimeter (Gore-Tex clothing), the samples presented show only a few capillary and macro-pores per square centimeter. This is not only not close, it is almost the opposite.
The waterproof effect of propolis
Propolis is known to have a waterproof effect. It is illustrated in the article by means of an image of a drop of water that is repelled by the propolis coating. Though, the image is only a snapshot. It shows a drop of water that was placed somewhere on the dry propolis coating. But what happens to a drop of water that is placed around one of the holes in the propolis over time?
I conducted an experiment with a simple setup that showed how a drop was soaked up by a wood sample covered with propolis within 2 hours. The water also partially evaporated. A second drop that was applied on an impermeable layer is visible on the left side of the image. Thereby, the evaporation that occurred can be estimated.
Did bees invent Gore-Tex?
Probably not. The physical background of this theory is partly wrong. All in all, I consider the theory of Torben Schiffer to be unsound.