WOOD: CHARACTERISTICS, TECHNIQUES AND INNOVATION

Rui Barreiros Duarte

Urbanism and Architecture Retired Full Professor

Lisbon School of Architecture

Principal Investigator from CIAUD

I - SIMPLE WOODS

This article refers to the characteristics, techniques and innovation that have been used in wood, which field of application has been extended beyond traditional uses, from simple to composites woods.

Being a natural material, it is well accepted for its qualities of comfort, environment, but also for its price and fast execution. It is important to take into account the importance of joints and gaps in the constructions, since they allow the transitions between materials and absorb the dilations resulting from their physical behavior.

The technical references presented correspond to a summary of essential aspects of the use and processing of the material, which must always be applied after the European quality standards have been complied, in order to avoid environmental and health problems, especially those arising from the use of chemical components.

CHARACTERISTICS OF WOOD

Since wood is a highly susceptible material, it must be taken into account in its use, its characteristics. The joints of the wooden boards result from their moisture content, their drying process and the way of cutting the wood from the veins it naturally has.
Its application on floors - Portuguese, English, skull, spine, open or layered -, must take into account this aspect, as well as in the parquets of blocks or plates.

The wood should be dried naturally, protected from water and moisture avoiding drying in autoclaves, which is a frequent solution but of questionable performance during the time of use. Its moisture content must never exceed 12%, with attention to these two factors, essential for measuring the quality of the material.

The type of tree growth should also be taken into account. The tree rings reveal the characteristics of its development process, each of them corresponding to a year of life.
If the wood has nodes, they should not be bumpy, avoiding these elements in the material to apply.

It is also essential to know the structure of the market, the marketing dimensions of the boards. Knowing what kind of savings can be achieved not only in the material selection, but also how many times the wood goes to the machine and the waste that is generated.
The choice of wood derives from its characteristics and behavior and must always be based on credible technical appraisals. In Portugal, the LNEC (National Laboratory of Civil Engineering) gives the certifications to the materials.

TRADITIONAL HARDENING OF SMOOTH WOODS

Traditionally, this type of treatment is obtained “by the application of the hydrofluosilicic acid, a Thomaz Cobley process, and consists of injecting into the wood a concentrated solution of lime, potassium and strontium, or one of the salts of these bases. It is still possible to make also with any metal salt the choice is indifferent once the solution employed forms an insoluble compound with the hydrofluosilicic acid, until the wood is perfectly impregnated and can withstand the influence of the flame. Thereafter, a second injection alone, with the referred acid, which becoming the first insoluble is fixed along with it in the pores of the wood.
It should be said that the latter must by any means be disentangled from the resin, before the operation described above is performed."¹

Research in this area has advanced towards achieving environmentally friendly results. Thus, we can mention a recent innovative method for hardening wood.

HARDNESS OF WOOD

“Wood is widely used in both interior and outdoor settings. However, because it is prone to the absorption and release of moisture, it is also susceptible to weathering. This occurs due to variations in the swelling of radial and axial grains. In cases where there is great variation, distortion of the material or fractures can occur. To prevent this, a number of tempering methods have been developed.

PROCESS PRINCIPLE

The most widely used techniques are thermal. These temper the wood through a relatively long period of heat treatment. The resultant “thermowood” comes into being as the structure of the wood is altered by steam or submersion in hot oil. The cell structure of the wood changes as a result and its propensity to absorb moisture, as well as its propensity for swelling, is markedly reduced. Nevertheless, certain volatile compounds such as resin and lignin are lost during the process, which leads to the wood exhibiting a reduced dynamic load-bearing capacity.

With this new method, which uses a natural wax solution, these undesirable effects are negated. The technique is based on the discovery of Hamburg-born master carpenter Jan Nies, who in 2009 was able markedly to increase the durability of wood by treating its surface with beeswax. Following the heat treatment, natural tree resin and wax were pressed into the wood cells and partitions by means of a pressure chamber. These were then allowed to harden during a controlled cooling process. The treated wood has a low shrinkage and swelling rate, as well great dimensional and formal stability.

APPLICATION

Tempering by means of wax impregnation is suitable for wood to be placed outdoors and to be used in street furnishings, playgrounds, and patios, and where it comes in contact with the ground.” ²

DURABILIY

The durability of the wood is its weakest point, which makes it an option to apply another material, especially in moister or abrasive areas. The durability products used in the past had pollutant and harmful properties. Today, research in this field seeks to find constructive solutions that are friendly to the environment.
The recovery of “chulipas” from the railroad to the construction has had a good feedback given the impregnation that the wood presents with oils of diverse content. It is also to be considered in traditional terms the treatment given to the wood used in the boats and their respective boards and to make an analogy with architecture.

"For the caulking, the tow is prepared in rolls, which will then be put into the joints with the irons - the “grafetos” - driven by means of machetes. When the hull is not new, cans of water are used to clean the remains of the previous caulk. Once caulked, the hull will be clutched with “pez louro”, to which is added little lamb fat to make it run better. This mixture is heated in iron pots of three feet, and applied with shotguns - a kind of brushes made with sheepskin. ³

TIMBER INCOMBUSTIBILITY

There are several types of fire behavior depending on the type of wood, but treatment with flame retardants should be done.

"With the application of ammonium phosphate, ammonia bi-borate and potash or soda silicate, it is possible to avoid the flammability of the wood.

These two first salts decompose with heat, the ammonia separates and leaves the phosphoric acid or the boric acid, which by fusion form a glaze on the surface of the wood.

This glaze prevents the interior of the wood from being in contact with the air, suspending the combustion.

As clarification I must say that the application of any of these salts is excessively expensive and I therefore advise the use of soda or potash silicate, applied by painting and at the concentration already indicated when dealing with the silicatization by the Kuhlmann process. This preparation is the one that offers more guarantees to avoid the combustibility of the wood." ⁴

INFESTATION

The quality control of wood must always be the one of the first things to do as the wood may be contaminated by hilotupus bajulos, reticutermis lucifugis, lictium, aneobium or European wood borer. The import process, when not properly taken care of, can introduce pest in the country. Lack of knowledge or laxity has an excessive negative impact on the economy, and there must be an effective system of control and monitoring.

II - COMPOSITE TIMBER

In composite woods, in order to avoid twisting/bending, the wood is used to form a crossed and blended pattern. When preparing the composite panels, thermal insulation is often inserted inside.

It is important to emphasize the difference between wood and wood pellets even if the finish does not identify the support, especially because of its behavior towards specific functions. It is always best to avoid the use of wood pellets in museums or houses, as it releases formalin - a chemical that attacks the eyes and metal parts, generating an environment that causes allergy.

                                                                 ---------------------FULL TEXT IN HOUSE DETAILS MAGAZINE NUMBER 1, APRIL2018-----------------------