The present invention relates to a method of producing through-coloured
wooden elements, particularly sawn wood products, and also to a through-coloured
By through-coloured is meant here that the wooden element has been
coloured or stained homogeneously throughout the wood. A sawn surface of the element
will therefore be coloured or stained regardless of the direction in which the
wood is sawn or cut. The term through-coloured is used in this significance hereinafter.
Coloured and stained wooden products become worn with time. Such wear,
gouges, lacerations and like impairments of the wooden products are particularly
noticeable because they expose the plain wood located beneath the coloured or stained
surface. Surfaces that shall be re-stained or re-coloured require a significant
amount of preparatory work, such as sanding, etc., which is very time-consuming.
The actual colouring process is also very time-consuming. These problems are solved
in accordance with the present invention with the aid of a method of through-colouring
wood as hereinbefore defined. Lacerations, gouges, marks and wear in and on the
wood are not as noticeable in this case. Furthermore, the wood can be sanded down
or worked whilst retaining the same colour throughout. The through-colouring process
is also less laborious than a manually performed colouring or staining process.
This affords significant benefits. Flooring materials and table tops are examples
of the areas in which such benefits are afforded. Thus, the invention enables the
production of through-coloured wooden floors and table tops which when showing
signs of wear need only be sanded down and then treated perhaps with an appropriate
oil or varnish. Another area of use is found in the wooden handles of cutlery.
Coloured or stained wooden cutlery washed in dishwashers quickly become disfigured
as a result of the stain wearing-off in the dishwasher. This problem is solved
when using through-coloured wood in accordance with the invention, since there
is no colour surface that can be impaired, by virtue of the fact that the wood
is through-coloured. It is now possible to produce wooden handles for cutlery that
withstand being washed in dishwashers.
Swedish Patent Application SE 9500689-6 describes a method of producing
impregnated wooden products. This method involves pressing a wooden element, or
wooden blank, isostatically in a so-called Quintus press or with the aid of some
other appropriate pressing method in a first stage of manufacture, at a pressure
greater than 800 bar, preferably greater than 1000 bar. The degree of compression
to which the wood is subjected will depend on the dryness, fibre direction, hardness
and other properties of the wood. Normally, this compression will result in the
reduction of one of the cross-sectional dimensions by between 20-50%, wherewith
the cells in said element are pressed together. The load on the wooden element
is then removed.
In a second step, the element is placed in a bath that contains an
impregnating agent, wherewith the element progressively swells as a result of liquid
entering into and being absorbed by the cells. The liquid penetrates the cells
such as to obtain complete impregnation of the element in its entirety. The extent
to which the element expands will depend, inter alia, on the material properties
of the element and its residence time in the bath. Pronounced expansion normally
takes place within the space of some hours and, in some instances, the material
returns to its original form within this time period. Suitable impregnating agents
are liquid fire retardants, liquid fungicides and glue.
The first two method steps may be supplemented with a third step in
which the element is again compressed isostatically with a pressure greater than
800 bar, preferably greater than 1000 bar, resulting in a hard element that is
fire-resistant, fungus-resistant and also resistant to deformation when glue is
The aforedescribed pressing process, which is also described in Swedish
Patent Application 9303821-4, raises the quality of low-quality wood by providing
a much harder end product. For instance, pine can be made as hard as oak, and oak
can be made twice as hard as its natural hardness. According to Swedish Patent
Application 9500689-6, it has been found that this pressing process makes it possible
to impregnate wood that could not earlier be impregnated. Such impregnation can
be achieved with large wooden products that are several square metres in area.
It has not earlier been possible to through-colour wood. However,
it has surprisingly been found that wood can be through-coloured in accordance
with the invention claimed in this Application by placing the wood in a colour
bath after the first isostatic pressing step. The colour penetrates into the wood,
which is then allowed to dry in a conventional manner, and may optionally be pressed
again. This colour penetration results in wood that is coloured throughout. In
addition, all the benefits of hardwoods are obtained by further pressing the wood
in accordance with SE 9303821-4, after through-colouring the wood.
According to the inventive method, a wooden element is pressed isostatically
in a first step at a pressure greater than 800 bar, preferably greater than 1000
bar, in a so-called Quintus press or by some other suitable pressing process, wherewith
compressing the wood cells in said element. This process is described in SE 9303821-4.
In a second step, the element is placed in a colour bath comprising, e.g., water
and a water-dissolved colorant. As the pressure-treated wood comes into contact
with water, the wood swells and draws water thereinto. The cellulose in the wood
and the water are both polar, whereby the water is readily absorbed by the wood.
The colorants are also polar and therewith accompany the water into the wood cells,
said colorants having a molecular size, i.e. a smallness, that enables them to
penetrate into the wood cells. The colorant shall not be excessively polar, since
this would cause it to stop at the surface of the wood instead of penetrating into
the wood. This also applies when the molecular size of the colorant is excessively
large and therefore unable to penetrate into the wood. The molecular size and polarity
of the colorant varies slightly, depending on the type of wood concerned. One of
normal skill in this art will be able to decide upon a suitable colorant experimentally.
This is achieved by through-colouring the type of wood concerned with different
colorants in combination with solvents, e.g. water, after having pressed a wooden
The solvent used will be one that is absorbed by the wood. Water is
a suitable solvent. Another suitable solvent is linseed oil, which has good penetrability
and wetability due to its small molecules. The colorant is dissolved in the solvent
and is sufficiently polar to accompany the solvent as it is absorbed in the wood,
and shall have a molecular size which ensures that it has "room" to accompany said
solvent into the wood. Azo dyes are suitable colorants in the present context.
Monomers, dimers or polymers of the azo dyes may be applicable, depending on the
molecular size of the azo dye or colorant and the choice of wood. The colorant
shall also have the polarity necessary for it to be able to enter the wood. The
skilled person is able to determine which colorant together with which solvent
will penetrate into the wood.
Woods that can be coloured in accordance with the invention include
hardwoods or leafwood, such as aspen and birch, and softwoods or coniferous wood
such as spruce and pine (redwood). The first step in which the wood is pressed
isostatically enables a large number of different types of wood to be through-coloured
in accordance with the invention.
A number of trials have been run using aspen and spruce which was
through-coloured with Herdins Akta Bets, which was made up by mixing sodium chloride
with an azo dyestuff according to the description of goods for genuine stain in
powder form having product number 102051 and obtained from Herdins Färgverk AB.
These trials will be described below.
The invention will now be described in more detail with reference
to the following examples. The inventive method comprises pressing a wooden element
isostatically in a first method step at a pressure greater than 800 bar, preferably
greater than 1000 bar, in a so-called Quintus press or by some other suitable pressing
process in which the wood cells in the element are compressed. The element is then
relieved of load. In a second method step, the element is placed in a liquid bath
containing a colorant dissolved in a solvent, such as water, wherewith water and
colorant are absorbed by the element. The colorant shall have a molecular size
which enables it to penetrate the wooden element and shall have a polarity which
enables it to penetrate the wood together with the water. Water and colorant migrate
into the wood as it swells. This takes place over some hours, and may be effected
at room temperature or at an elevated temperature of up to 100°C. The through-colouring
process proceeds more quickly at temperatures above room temperature. Water containing
colorant penetrates into the wood clement as the wood swells, thereby resulting
in a through-coloured element.
The element is then dried conventionally.
It has also been found that it is possible to through-colour wet wood
or raw wood. In a first step of isostatically pressing the raw wood at a pressure
greater than 800 bar, preferably greater than 1000 bar, the lumina are opened up
or some properties of the wood are changed. The opened up lumina will be able to
absorb the colorant and the solvent, in a later step, which makes the wooden element
through-coloured. In a later step the wooden element is placed in a chamber and
the colorant and the solvent is delivered. This later step can be done with a conventional
pressure impregnation process, with vacuum and/or pressure. This step is necessary
since the wood keeps a quite high content of sap. The pressure and/ or vacuum step
makes the sap already contained in the wood able to be exchanged against the colorant
and the solvent.
It is even possible to through-colour wet or raw spruce, which has
been known to be impossible to impregnate.
Subsequent to having through-coloured the wooden element, said element
may be pressed isostatically at least one more time in a third step, at a pressure
greater than 800 bar, preferably greater than 1000 bar. This results in a through-coloured
wood that has the hardness of a hardwood.
The wood through-colouring process can also conceivably be combined
with the impregnation process described in SE 9500689-6, therewith incorporating
a fire retardant, an anti-fungus agent or glue into the through-coloured wooden
The invention also relates to a through-coloured wooden element that
has been through-coloured in accordance with the aforedescribed method.
A piece of aspen wood measuring 7x4.5x15.5 cm was pressed isostatically
in a Quintus press (in accordance with the method described in SE 9303821-4). The
wood was pressed at 1300 bar for two minutes at room temperature.
A colouring bath was prepared by mixing HERDINS ÄKTA BETS, No. 59,
ebony black in powder form with water, in accordance with the instructions on the
packet (without ammonia). The solution was brought to a boiling point and the pressure-treated
wood piece then placed in the solution and kept there for about two hours while
boiling the solution, after which the wood was removed and dried. The wood was
then sawn in two, whereby it could be seen that the piece of wood was coloured
A piece of spruce measuring 4x4x5 cm was pressed in a second test
in the same way as that described in Example 1. The colouring bath was prepared
in the same way as that described in Example 1 and the wood was through-coloured
in the same way with the exception that HERDINS ÄKTA BETS, No. 51, red, was used
instead. The wood was removed from the bath and dried and then sawn in two, from
which it could be seen that the wood had been coloured throughout.
The present invention provides a unique method of through-coloured
wood that has a wide area of use. Practically all types of wood can be treated
in accordance with the invention. In addition, the invention can be applied to
produce through-coloured wood of enhanced hardness. The through-colouring process
is not so laborious as painting or staining wood by hand. As previously mentioned,
the areas of use include flooring materials and table tops, and also wooden panels,
wooden stairs, kitchen elements, glued wooden beams, shafts, sport equipment, etc.
When such wooden products and articles become damaged and worn, their surfaces
can be sanded down to expose new, blemish-free coloured surfaces that do not require
re-colouring. Damage and wear to the surface of the wood will not be as noticeable
as would otherwise be the case, since the wood is through-coloured and consequently
no "white" wood will show through a damaged surface. It is also possible to produce
cutlery with stained wooden handles that can withstand being washed in a dishwasher.