# Recent Advances in Bio-Based Wood Protective Systems: A Comprehensive Review
## 5 Bio-Based Pigments and Dyes
### Natural Pigments  
Contemporary society is realizing that industrial waste frequently holds untapped potential that can be valued and utilized for other purposes. This principle applies to various industries, including wood coatings, where there is a growing inclination to seek novel pigments and additives. These components not only impart distinct colours to coatings, but also align with environmental considerations and the principles of a circular economy. Considering this perspective, a recent investigation explored the pros and cons of utilizing pigment from spirulina as a natural colourant for wood treatment solutions [42]. Spirulina (_Spirulina platensis_) is a type of cyanobacteria acknowledged as one of the most prominent microalgal reservoirs for the industrial synthesis of phycobiliproteins [140; 141], encompassing light-harvesting protein pigments. The study highlighted the impressive colour effects achieved with the introduction of a spirulina-based additive. Nevertheless, the natural pigment showed notable issues related to its susceptibility to UV-B rays, which could lead to the degradation, as well as the potential fading, of the phycocyanin part present in spirulina. As a result, research underscores the significance of adopting suitable precautions to protect bio-based pigment from external elements, like temperature, sunlight, and liquids. Therefore, from this standpoint, a subsequent study aimed to uncover the combined effects of two natural additives on the endurance and protective characteristics of a bio-based wood paint [41]. While spirulina is utilized as an organic pigment, carnauba \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Filler/Additive** & **Properties** & **Results** & **Ref** \\ \hline Essential oil from _Pelargonium gravelens_ & Increased fungal inhibition & 100\% fungal inhibition against _C. putetana_ & [121] \\ Scots Pine Knotwood & Increased fungal inhibition & Fungal growth inhibition up to 67\% against _G. trabecular_ & [123] \\ Citric acid oligomer & Increased fungal inhibition & Up to 15 mm associated with the zone of inhibition against _S. aureus_ & [125] \\ Polybasic carboxylic acid & Improved flame & LOI value up to 30.7 & [129] \\  & retardant properties & Peak to heat release rate reduction from 193.2 to 150.3 kW/m\({}^{2}\) & [134] \\  & Improved fire resistance & Reduction in the specific extinction area of 44.12 m\({}^{2}\) kg\({}^{-1}\) & [135] \\ \hline \hline \end{tabular}
\end{table} 
Table 3: _Cont._was serves as a versatile filler. The analyses demonstrated the impressive colouring capability of spirulina, imparting a distinct green hue to the paint and enhancing its reflective properties. Conversely, the wax influenced the surface texture of the sample, heightening its roughness and diminishing the coating's glossiness, as highlighted in Figure 4, which reveals the appearance of the different samples. Consequently, these two bio-based additives substantially transformed the coating's visual attributes, modifying both its colour and reflective traits. Furthermore, the inclusion of wax in this scenario led to an improvement in the water resistance of the coatings, implying enhanced barrier properties of the layer. Additionally, it introduced a reinforcing effect within the coating, potentially reducing the risk of mechanical wear due to abrasion. Ultimately, spirulina and wax emerged as two intriguing natural additives for wood coatings, whose combined action can offer diverse aesthetic enhancements and boost the durability of wood finishes. However, a recent investigation has also assessed the coloration impact of an eco-friendly black pigment sourced from discarded wood materials and incorporated into a bio-based wood paint [38]. The pigment was created by utilizing wood waste originating from industries such as paper, lumber, furniture, and flooring. To produce the pigment, the wood underwent a heat treatment within oxygen-free chambers. This controlled environment prevented the wood from combusting and releasing CO\({}_{2}\) into the atmosphere.