# Laboratory Investigations of Mold Growth on Transverse and Longitudinal Wood Surfaces
## 1 Introduction  
Wood is a natural, renewable material that has been used in building structures for many centuries. The durability of wood is affected by a number of loads, e.g., weathering, chemicals, and biological agents. Wood can last for hundreds of years in dry conditions [1]. Under risky conditions of increased moisture, mold fungi are amongst the first colonizers of wood. Although molds do not cause significant structural damage to wood, they are a clear indicator showing that the moisture state of a wooden element is problematic.  
The durability of wood products is an urgent issue for its future use as a competitive building material. The occurrence of molds on wooden building elements may increase in the future due to changing climatic conditions [2; 3]. Faster drying methods, and the wrong use of wood in construction without regard to its natural durability limits, may also have an effect on mold growth on wooden elements.  
The remediation of mold damage and the renovation of wooden elements is expensive and technically demanding, especially in the case of building structural elements (e.g., timber framing, roof trusses). For these reasons, the prevention of mold growth has economic benefits. Several mold growth prediction models [4; 5; 6; 7] have been developed in the last decades to predict the risk of mold growth on wooden elements in buildings.  
Whereas mold germination is solely related with relative humidity and temperature, mold growth depends on both environmental conditions and the availability of nutrients.  
The concentration of simple carbohydrates on the surface affects the growth rate of fungi [8]. The distribution of nutrients can be affected by processing wood prior to experiments [9; 10].  
The minimum relative humidity for mold growth on wood is between 75 and 80% [11]. In such environmental conditions, the time for the first signs of molds (microscopic observation) takes many weeks [12; 13]. The growth rate decreases with the decrease in relative humidity [11; 12; 13; 14; 15; 16]. The mold development also strongly depends on the duration of suitable exposure. If relative humidity fluctuates between favorable and unfavorable environmental conditions, mold development slows [4; 9; 17]. In stable conditions, the time required for the first visual occurrence of molds on pine and spruce sapwood at temperatures between 25 \({}^{\circ}\)C and 40 \({}^{\circ}\)C and relative humidity exceeding 95% is only a few days [18]. In addition to environmental conditions, wood species, surface quality and treatment, and differences between heartwood and sapwood, influence mold growth as well. For example, pine has been shown to be more susceptible to fungal growth than spruce [8] and the growth is faster on original kiln dried wood than on sawn wood [5]. Other influencing factors are felling time and the drying method [10].  
This paper presents mold growth curves observed during a laboratory experiment on pine and spruce wood. The experiment was carried out under constant environmental conditions (temperature 22 \({}^{\circ}\)C; relative humidity 75, 87, and 95%). The wood specimens were prepared with respect to the structure of the wood to obtain specimens showing tangential, radial, and transversal surfaces. Mold growth was recorded over time on the basis of regular microscopic and macroscopic observations. The focus was primarily on three important characteristic times in mold development: the time of the first microscopic signs of the growth, the time of the first macroscopic signs of growth, and the time when the maximum coverage was achieved. When appropriate, mold decay was also quantitatively expressed. The differences between mold growth on anatomical sections, and the differences between the data obtained by the experiment performed and the mold growth data found in the literature are discussed.