Reverse engineering is essentially the process of discovering the original materials and technology used to create a component [29]. With 3D scanning the potential to recreate the whole or a part of a com- ponent is made possible. Research concerning 3D modeling of the ob- jects have been comprehensively conducted [36-38]. Most scanners, however, produce objects data of accuracy and resolution beyond the ability to practically reproduce them in physical forms [30]. Reproduc- tion devices such as 3D printers are not capable of maintaining this res- olution in the final product. Wachowiack and Karas [29] state, “the term resolution applied to replication refers to the ability to accurately pro- duce an object from scan data” (p.144). Minimizing the gap between scanning accuracy and reproduction resolution is a pervasive challenge for building components. Ultimately, however, the level of resolution and choice of material will determine the cost of restoration or the rep- lica that is produced.

2.2. Physical reproduction: 3D printing for construction

3D printing (also referred to as Additive Manufacturing/Rapid Prototyping) has been extensively used in the manufacturing sector as a way to automate, accelerate production and reduce material waste [39]. The use of this technology enables a variety of objects to be manufactured or constructed direct from a digital environment; this is, however, dependent on the specification that is provided to the print- er and there are no limitations with regard to the material being used. As this technology has matured, it has become reliable and cheaper and is now being used in the construction industry; referred to as 'Addi- tive Construction' [40], though its application in practice has still been limited [41–43]. A detailed review of 3D printing and its potential appli- cations in construction can be found in Lim et al. [42], Perkins and Skitmore [39], Wu et al. [44] and Labonnote et al. [40]. Notably, those techniques that can be used for Additive Construction are:

a) Concrete-layered overlay: this method is based on layered concrete extrusion and curing prototyping through a process of self-stabiliza- tion. Contour Crafting (CC) can be undertaken, which allows a smooth surface akin to a trowel to printed [45]. With this method it is possible to erect a square foot of wall within 20 s and a 200 m2 single-storey house in one day [43]. Concrete Printing, another similar technology, is capable of smaller depositing resolution and incorporates functional voids into a bench structure  [42];

b) Sand powder-layered adhesive stack: this method refers to the pro- cess of layering a selectively bonding and solidifying powder using an multi-hole adhesion agent extruded, such as D-shape developed by Monolite Ltd. Hardened powder such as this can be used to con- struct stone-like heterotypic components or structures [45,46];    and

c) Mechanization: this is quintessentially an automated or robotic pro- cess, usually realized by a robotic arm, which utilizes different mate- rials from above two methods (e.g., brick, metal and plastics). Gramazio et al. [47] introduced the concept of robotic fabrication whereby bricks are produced and stacked to form freeform struc- tures. A Dutch startup known as MX3D [48] builds a steel-truss bridge automatically by two robotic arms.

Despite the emergence of 3D printing and several successful experi- ments being undertaken during construction, there have been no studies that have examined 3D printing in the context of full scale re- production of ornamental stony components for historical buildings. This paper aims to fill this gap and proposes a novel digital reproduction process, specifically focusing on historical building ornamental components.

3.