The use of oil in wood has long been practiced and is continued up to date. And with lots of oil types available on the market, it is always an advantage to be equipped informatively. Amongst the many oils out there, Danish oil and linseed oil are two of the most well-known.
Getting to know the characteristics of each will give us a clear view of their functions and where they can be used most appropriately.
This article will inform you what should be known about these two popular oils for wood before putting them to good use in your next project! Topics are fragmented as follows:
- Wood Protection
- Oil for Wood
- Fire Safety for Oils
- Linseed Oil: Source and Variations
- Origin of the Term Danish Oil
- Composition of Danish Oil
Properties of Danish Oil in the Market
I. Wood Protection
Wood furniture needs to be protected against three major things: water, microorganisms, and fire. Since wood is naturally porous, water can seep through it and cause the wood to swell when wet and shrink upon drying. Wood can also be attacked by microorganisms such as fungi, bacteria, and algae, which cause its degradation.
Meanwhile, wood is flammable and to prevent it from combusting or spreading fire, it must be treated in accordance to fire safety. Fortunately, simple application of oil to wood can address these factors and thus protect wood furniture.
II. Oil for Wood
Natural oils used as wood coatings are usually extracted from plant seeds and are classified into drying oil, semi-drying oil, and non-drying oil. Linseed oil as well as the usual components of Danish oils are drying oils.
These oils dry naturally in the presence of oxygen in the environment without the need for treatment or additional chemicals. As a liquid, when applied to wood, it penetrates the spaces within the wood as well as coats the wood surface. Upon drying, it creates a solid film that becomes a barrier for the wood against the likes of water and microorganisms.
Different types of oils have different types of chemistries and different effects on wood coatings. Before delving into the different effects of our two main oils, let us first look into their characteristics.
III. Fire Safety for Oils
Fire safety information is a vital point to know before using either linseed or Danish oil as both are susceptible to spontaneous combustion. Fire requires three components to start: oxygen, heat, and combustible material (fuel).
The reaction of drying oils with oxygen produces heat. Thus, two of the requirements for fire are already met with oxygen being present in the environment. Normally, oil applied on wood is spread as a thin film, hence the heat dissipated from the reaction with oxygen is not enough for it to catch fire even if wood serves as a combustible material.
The cloth used for application as well as rags used for cleaning up, on the other hand, are soaked with oil and as they generate heat from reaction with oxygen, the fabric acts as the combustible material and the system poses a hazard to combustion.
Accordingly, proper disposal of materials used with drying oils must be observed. Also, as different oils have different autoignition temperatures, it is important to note the suggested working temperatures of each brand.
Linseed oil: Source and Variations
Linseed, also known as flax, is a flowering plant grown in temperate climates. As a plant, it is most commonly referred to as “flax”, while “linseed” is mostly used to refer to the oil extracted from the plant.
Its fiber-rich stem is used in the textile industry while its seed is the source of food and oil production. Due to technological developments in fiber production and the rise of synthetic coatings for wood around the 1950s, linseed plantation was significantly reduced in the United States, though not completely lost.
In recent times, linseed oil is gradually regaining its popularity for its health benefits. Currently, its plantations are concentrated in North Dakota and Minnesota.
Flax pods contain the seed from which linseed oil can be extracted. Around 35% to 45% of the flax seed is oil-containing. This percentage varies with the method of farming and the climate in which the crop was grown.
Raw linseed oil extracted from the seed is in its purest form. This untreated oil is classified as a drying oil which means it dries to a solid film naturally with time. The component of oil which causes this drying is the unsaturated fatty acids.
Chemically speaking, they are the double bonds in the fatty acid carbon chain. The double bonds correspond to the degree of unsaturation of the fatty acid. A higher number of double bonds means a higher degree of unsaturation and consequently, faster drying time.
The drying of raw linseed oil is, however, a slow process. When applied directly to wood, it can take up to 10 weeks to completely dry. Needless to say, the film during this waiting time is tacky and is highly prone to dirt accumulation. Owing to this great disadvantage, various treatments were developed to improve the drying time as well as other properties of linseed oil.
Types of Linseed Oil
Blown Linseed Oil introduces air into the oil while subjecting it to a temperature between 40 to 150 degrees C. No other chemical is added to the oil. Deliberate exposure to air allows the oxidation process to occur within the oil, thus shortening its effective drying time compared to raw linseed oil.
Drying time reduction depends on the duration of “blowing,” the amount of air introduced, and the temperature used. This treatment takes time and the endpoint is considered when the desired increase in viscosity is achieved. Also, the procedure itself is old-fashioned and such a product is no longer visibly available on the market.
Boiled Linseed Oil is produced with slight variation from the blown process. The blowing of air may or may not be introduced into the system, while the addition of chemical drier (metal oxides, carbonates, acetates) is a must.
Its main treatment involves heating the oil between 150 to 250 degrees C. The term “boiled” linseed is actually a misnomer as the process does not really boil the oil. Raw linseed oil boils at a higher temperature, about 316 degrees C and above. The drying time of boiled linseed oil drastically shortens to around 24 hours compared to the week-long duration of raw linseed oil.
Also, this treatment allows the introduction of different colors for aesthetic options. This type of treated linseed oil is very active in the market and is available under different brands.
Heating linseed oil to a temperature between 270 to 310 degrees C without the addition of driers and blowing of air makes Stand Oil (Bodied Oil or Polymerized Oil). This process shortens the drying time of raw linseed oil, improves its toughness, and reduces film yellowing.
However, this procedure increases viscosity, which is not suitable for coating applications. Hence, for practical use, stand oils are mixed with other materials such as solvents for workability.
Origin of the Term Danish Oil
Unlike linseed oil, which literally means oil from linseed, Danish oils do not necessarily come from Denmark. During the 20th century, furniture made in Scandinavian countries (such as Denmark) rose to global popularity.
Danish wood furniture is protected by a blend of oils with a characteristic satin finish. As the design of furniture became well-loved, the oil used to produce such a finish gained prominence and began to be called “Danish oil.” Presently, Danish oil is characterized mostly by its satin finish and not by its country of origin.
Composition of Danish Oil
Finish and protective function are the key factors regarded for Danish oil. For this reason, its exact composition was not properly established. Different manufacturers use different materials or blends to achieve the traditional finish plus the desired property they want to incorporate in their version of Danish oil.
Usually, they are made up of natural oil such as linseed and/or tung oil. Other materials such as synthetic resin, solvents, or driers may be added to modify their properties.
Linseed oil is thoroughly discussed above. Hence, let us look into the properties of tung oil in the following. Tung oil is a natural oil derived from the seeds of tung trees. It is the most reactive drying oil, having a higher degree of conjugation in its structure.
Double bonds separated by a single bond define conjugation. Tung oil, though having a comparable amount of double bond with linseed oil, has a higher degree of conjugation and this corresponds to the faster drying time of tung oil versus linseed oil. It has other advantages over linseed oil, like creating a clearer film compared to the yellowish tinge of linseed oil. It also produces a harder film, making it more durable and less resistant to water.
However, tung oil is more expensive than linseed oil.
Properties of Danish Oil in the Market
In this section, different brands of Danish oil available in the US market are tabulated to give us a quick view of the difference in properties and use of each product.
Table I. Properties of Danish Oils available in the US market
|Brand Name||WATCO||TRIED & TRUE||FIDDES||FURNITURE CLINIC|
|Composition||natural drying &
semi-drying oils, rosin ester, mineral spirits, pigment
|pure polymerized linseed oil||tung oil, resin & solvent||natural oil & resin|
|Applied by||brush or cloth||cloth||brush||foam brush or cloth|
|Drying Time||6-8 hours (touch)
10 hours (handle)
72 hours (recoat)
|8 hours||4-6 hours||4-6 hours|
(requires finishing coat for protection)
|interior woodworks as sealer and finish
(including food utensils)
|interior and exterior woodworks as sealer and finish||interior and exterior woodworks
(including food utensils)
|Finish||different colors available||sheen*||low sheen||satin|
*sheen increases with sanding
None of the four Danish oil brands above are exactly the same and thus their use and properties differ too.
Watco® Danish Oil, the most popular brand, has the greatest number of materials blended to come up with the finished product. The variations brought about by the mix allow Watco® Danish Oil to carry pigment and provides a wide range of color for wood decoration.
It is, however, focused on artistic factors and protection of the wood from the inside. Thus, it recommends subsequent coating to protect the surface of the wood.
Tried and True Danish Oil, on the other hand, shows that linseed oil and Danish oil can actually be the same thing! This product contains pure linseed oil which was treated at high temperatures to increase the degree of polymerization and speed up the drying time.
No additional chemicals were added in the process, allowing the oil to be applied to wood utensils for food. Fiddes Danish Oil demonstrates the effect of tung oil as it has a shorter drying time and durability for exterior application.
Furniture Clinic Danish Oil has similarities with Fiddes Danish Oil and from this, it can be deduced that tung oil is the natural oil present as its component.
Undoubtedly, the blending of two oils can get the best of both worlds and this makes mixes such as Danish oil special as they are modified to cater to specific needs. However, having no restrictions on the contents of Danish oil, it is therefore imperative to look into the properties of a specific brand to understand its content and where it is best intended to use for.