What is Reactive Stain?
Reactive stain is a wood processing technique that utilizes chemical reagents to darken the color of wood. This technique is commonly employed in the production of wood products, including flooring. Reactive stain can give wood a gray color that appears aged and darker, which has gained significant market value due to the widespread popularity of gray tones in interior color design.
In comparison to traditional color lacquer coatings, reactive stain offers a more natural and aesthetically pleasing stain with higher color saturation. Moreover, it preserves the original texture detail and provides a greater sense of layering. Therefore, it is extensively used in the production of engineered wood flooring.
How do Reactive Stains Work?
Polyphenols present in the wood, such as tannins and flavonoids, react with iron ions to form an insoluble dark complex substance, resulting in a dark gray color and a modern antiqued appearance. At the flooring factory, color-changing chemicals can be introduced to the veneer surface by impregnating or applying them with an iron salt solution. Afterward, the veneer is dried to the standard moisture content to achieve a color-changing surface suitable for stained engineered flooring.
The staining effect may vary depending on the tannin content of the wood species. For instance, European oak has a stronger staining effect compared to American white oak. Furthermore, the distribution of tannins in the same piece of wood may also differ. Sapwood, with a lower tannin content, typically exhibits a lighter staining than heartwood. Moreover, the reactive staining process does not produce as uniform a finish as traditional paint staining techniques.
Safety Concerns about Reactive Stains
There is no need to be concerned about the potential health hazards of the chemicals used in reactive stains. The coloring agents used in reactive stains are common metal salt solutions, such as ferrous sulphite solutions, which are considered non-toxic. Additionally, the dark complexes created by reacting tannins from wood with iron salt solutions are the primary component of blue-black ink commonly used in paintings and calligraphic relics.
Reactive staining is a safe and widely used solution for antiquing flooring in Europe and North America. Many customers opt to customize their desired flooring color by manually applying the color-changing reagent on unfinished flooring planks.
Stain Quality on Engineered Flooring
- Color Hue
The commercial value of flooring is determined by the color after reactive staining. However, the actual staining outcome depends on the wood species and the distribution of tannins within the wood.
To achieve controlled reactive staining, the stain’s formulation and concentration need to be adjusted for different species to obtain chemically stained wood in hues of gray, blue, black, and purple. Chemical precursors such as flavonoids or phenols can also be applied to the veneer surface to achieve consistent discoloration between heartwood and sapwood, earlywood and latewood.
- High Permeability
In the stained flooring, the reaction does not only occur on the wood surface but should remain uniformly stained even after edge cutting or surface grinding.
Wood is a porous structure, but the pores are not fully connected. It is still essential to improve the penetration of the chemical agents into the wood with some special treatments, including vacuum pressure impregnation or high-temperature steam pretreatment, which have been widely used in the preservative-treated timber industry. Pre-treatment opens up the channels between vessels and allows the stain to penetrate into the wood, resulting in a deeper and more uniform reactive stain inside the wood.
- Color Fastness
Migration, oxidation, and photo-aging are the primary causes of color fading in stained wood, which is caused by chromogenic substances that produce color. In practical applications of stained engineered flooring, various factors can cause unpredictable color changes, which significantly affects the appearance and commercial value.
To prevent the loss of chromogenic substances inside the wood, the polyphenol iron complexes produced during the staining reaction are relatively stable and chemically bond with the wood cell walls to reduce migration. Additionally, a color-fixing agent added during veneer surface treatment effectively reduces the loss of chromogenic substances and moisture migration in the wood.
Both wood and polymer coatings are inevitably subject to gradual photo-aging and fading. Studies have shown that the chromogenic substances are more light and oxidation-resistant than the wood itself. Moreover, UV absorbers can be added to coatings to reduce the impact of light on the flooring.
