## 1 Introduction  
Our stone cultural heritage is subject to weathering over the years. This deterioration is due to physical, chemical and biological factors acting synergistically that cause disfiguration and dissolution of the stone, an increase in porosity and overall weakening of the structure [1]. When the building is of historic and cultural importance, it is necessary that careful restorative action be taken, with the aim of returning the monument, as much as possible, to its original condition. This may involve removal of unwanted surface deposits (cleaning) and strengthening of degraded parts of the structure (consolidation), and ultimately replacement of blocks or whole sections of the structure. Several processes, such as microbial transformation of stone minerals [1], discoloring by microbial pigments including algal, bacterial and fungal biomolecules with staining properties [2, 3, 4, 5], and atmospheric pollutant deposition [6], which may all operate simultaneously at surfaces, can make cleaning procedures challenging. Furthermore, cleaning can induce irreversible damage; thus a cost-benefit analysis should be considered in a prior assessment phase, in order to reduce potential risks to artwork [7].  
The consolidation and conservation of these buildings requires the use of materials that are compatible with the original structure. Consolidants that have been used in the past, or, indeed, are still in use, include those based on lime water, which is not very efficient [8], and silicic acid, which can lack the necessary flexibility for resisting stresses within the stone [9]. Nanoconsolidants, used as recent improvements in consolidation strategies [10] include the use of nanoline [11] and nanosilica and may be more effective in reducing porosity [12, 13]. Nanoline doped with calcium hydroxide/zinc oxide quantum dots has been shown to form a more durable conservation treatment for limestone than silica-based consolidants [14], but it is more costly. The disadvantages of traditional consolidants, for both calcareous and siliceous stone, are poor performance, structural drawbacks such as low adhesion, poor penetration and flexibility and, in the case of organic substances such as alkoxysilanes, the necessity to be dissolved in volatile organic compounds (VOCs), which carry environmental and human health risks [15].  
Synthetic resins, such as silane, epoxy, acrylic and polysiloxane, plug the pores when they polymerize, thus causing water retention and internal degradation, while external coatings can peel off. In order to increase the efficiency, several chemical modifications of old consolidants have been produced [16]. However, perhaps more promising are the new, biologically-based consolidants (bioconsolidants), which do not rely on synthetic components, but are naturally-occurring structural plant biopolymers [17; 18], or utilize the enzyme activities of microorganisms to produce new materials.  
Not only consolidants, but other traditional conservation treatments, can be replaced by more environmentally acceptable, biologically-based, measures to restore stone. This review seeks to provide updated information on these innovative conservation treatments that have been or are being developed.