## 1 Introduction  
Wood, a fundamental natural resource, has been employed in various applications since time immemorial. Its versatility, renewable nature, and wide availability have rendered it an indispensable material in industries such as construction, furniture, and manufacturing [1; 2; 3]. However, one of the perennial challenges associated with wood products is their susceptibility to combustion. The threat of fire not only poses safety concerns but also has substantial economic and ecological ramifications. Hence, there has been an enduring quest to enhance the fire-resistant properties of wood [4; 5; 6; 7; 8; 9; 10]. Among the numerous wood species available, poplar wood (_Populus_ spp.) stands out for its fast growth rate and ease of cultivation. Polar is widely distributed across temperate regions, and its utilization has surged in recent years, particularly in applications where rapid growth is essential [11]. Black poplar (_Populus nigra_ L.) is one of these species. Black poplar is distributed in North Africa, Central and Western Asia, and Europe, especially in wetlands along riverbanks [12]. The distribution of poplar species in the world is more than 100 million ha. Turkiye ranks fourth in the world in terms of poplar plantation area [13]. More than 3 million m\({}^{3}\) of wood are obtained annually from this species alone in Turkiye [14]. Although black poplar wood is widely used in furniture production, it can also find a place as a raw material in the packaging industry (boxes, crates, pallets, etc.) and in the production of models, plywood, matches, composite panels, and prostheses [15; 16]. Nevertheless, like many other wood varieties, poplar wood is inherently vulnerable to fire, necessitating innovative approaches to improve its fire-resistant characteristics. In this context, the impregnation of wood with fire-retardant chemicals has emerged as a promising avenue for enhancing its fire resistance. Impregnation involves the penetration of wood with fire-retardant substances, which can alter the wood's surface properties and combustion behavior [17; 18; 19]. The choice of impregnation technique and the type of fire retardant used are important factors in determining the effectiveness of this process. Therefore, it becomes imperative to explore the influence of diverse impregnation techniques on the surface characteristics and combustion behavior of poplar wood [20; 21; 22; 23; 24; 25]. The practice of impregnating wood with various substances to enhance its properties is a method that has been used for many years. It has been employed for centuries, albeit with rudimentary techniques. Modern wood impregnation techniques have evolved significantly [26; 27; 28; 29; 30]. One of the earliest methods involved simply soaking wood in a solution containing fire-retardant chemicals. While this method is straightforward, it often results in uneven impregnation and inadequate penetration of fire retardants into the wood's cellular structure. To address these limitations, vacuum impregnation and pressure impregnation techniques were developed. Vacuum impregnation, in particular, involves subjecting wood to reduced pressure before immersing it in a fire-retardant solution [24; 31; 32; 33; 34]. The choice of fire-retardant chemicals is a critical determinant of the efficacy of wood impregnation. Fire retardants can be categorized into several classes, including inorganic compounds, organic compounds, and intumescent agents [35; 36; 37; 38]. Inorganic fire retardants, such as ammonium phosphate and aluminum hydroxide, work by releasing water vapor when exposed to heat, thereby reducing the wood's temperature and retarding combustion. One of these, calcium oxide (CaO), is a white, corrosive, and alkaline solid [39]. Calcium oxide is used in the construction industry and in the production of paper, among many other applications, such as the manufacture of various types of glass [39; 40]. These compounds are known for their non-toxic nature and widespread use in wood impregnation.