The following characteristics are required of such materials: low humble density, flexibility, thermal properties similar to wood, water-repellency, with sufficient mechanical strength and resistance to external factors. Moreover, such material should be cheap, easily achievable and easy to install. Due to these rather complex requirements, the choice of material, from which a seal can be made, it is definitely not easy. So far, there is no material meeting the above-mentioned requirements.
Until recently, concrete was widely used for seals, however, besides a few advantages, it has so many disadvantages, that its use was definitely abandoned. The most important disadvantages of concrete are: a lot of weight, causing the seals to slide down and create gaps, bad thermal properties, conducive to the condensation of water vapor on the inner surface and the dampness of the cavity, inelasticity leading to cracks in large seals due to concrete shrinkage or tree movements; low durability due to water absorption and the associated cracking and crumbling in winter.
Various materials joined with a bituminous binder turned out to be much better fillers than concrete, such as lightweight aggregates, wood sawdust and others.
The bituminous binder makes the seals more flexible, better thermal properties, and sometimes also lightness. Building in bitumen is easy, since some of the binders used in their manufacture contain fluidizing agents and require nothing to be heated. This makes it possible to prepare a seal with sufficient cold plasticity. The gradual hardening of the binder occurs after the fluidizing agent has evaporated, usually after a few hours.
There have also been attempts to use various foam-type plastics for this purpose.
Difficulty in getting the right material as a filler, low sealing efficiency, as a way of preventing decay of wood and improving the mechanical properties of the trunk, led to almost complete abandonment of sealing.
Therefore, more and more often places of large defects are left open after proper cleaning and protection.
If it is necessary for aesthetic reasons, the cavities of the cavities are covered with thin shell screens (often made of reinforced concrete slabs), which, properly attached to the edges of the cavity, can overgrow with healing tissue.