Medal Project

GLOSSARY

Alloy                             A solid solution (mixture) of two or more elements, at least one of which is a metal, and characterised by metallic resulting properties. The ancient alloys are characterised by a more complex micro-chemical structure with respect to the modern ones. For example, the Cu-based alloys were generally produced first via a casting process and then, subjected to cold and/or hot hammering and heating cycles for obtaining the final shape. Their resulting microstructures can be characterised by a dendritic structure and tin inverse segregated areas. Furthermore, twinning, slip lines and deformed crystals, with inclusions and copper and/or copper-iron sulfide particles at grain boundaries, can be present. This complex microstructure plays a determining role in the formation of the corrosion products.

                  

                          

Amalgam                        An alloy of mercury with another metal or other metals such as gold, tin, silver, zinc and lead.

Annealing                       A multi-phased process of heat treatment applied to a metal or alloy to relief of internal stress caused by mechanical working and to induce homogenisation of composition and softening of a metal hardened by cold working. The annealing process alters the micro-structure and influence the grain size, the higher the temperature, the larger the grain will grow. In ancient times, the annealing process was carried out on a fire of wood or wood charcoal.

                             

Banding                          Micro-structural feature related to local difference of chemical composition or to the occurrence of segregation phenomena. The bands themselves could be evidenced by the preferential alignment and not homogeneous distribution of alloying elements that could be aligned on filaments or plates parallel to the direction of working.

Bloom                            Bloom is the roughly finished iron spongy mass produced in a solid condition directly by the reduction of iron ore (hematite or magnetite) in a bloomery furnace under a carbon monoxide atmosphere. The carbon content in the bloom could be variable and contain a large amount of small inclusions. The crude bloom must be worked and shaped at high temperature to sinter the iron and to eliminate the residual slag.

Bronze disease                One of the most serious hazards of bronze is the cyclic degradation of copper, romantically defined as “bronze disease” whose degrading agent is chlorine that forms copper chloride.
Copper chloride (CuCl,  nantokite) when exposed to moisture cyclically react with oxygen and water from  atmosphere and gives rise to the formation of Cu₂Cl(OH)₃ (copper oxy-chloride, atacamite, botallackite, clinoatacamite, paratacamite) and hydrochloric acid. This latter reacts with copper to form new cuprous chloride and, in this way copper, chlorine, oxygen and water are converted in cuprite (Cu₂O) and atacamite or other its polymorphs in cyclical and continuous process according to the following reaction:

                                4CuCl + 4H₂O + O₂ => CuCl₂ ^. 3Cu(OH)₂ + 2HCl

                               2Cu + 2HCl => 2CuCl + H₂

“Bronze disease” products are small patches of light green powdery voluminous basic chlorides of Cu, which disrupt the surface and may disfigure the artefact or eventually disintegrate it via a progressive corrosion.

From a conservation point of view, it is worth pointing out that the removal of the patina layer composed by Cu (II) compounds and cuprite (Cu₂O) from the surface of the bronze object could expose the copper chlorides present under the Cu₂O layer thus inducing the interaction between copper chloride, humidity and oxygen and then, the cyclic reaction of bronze disease can start causing the partial destruction of the artefact.

As a consequence, particular attention must be paid during the removal of surface encrustations and corrosion products layers in order to avoid the exposure of copper chloride enriched regions.

           

Casting                           The operation of pouring metal into a mold where solidification occurs. The mold could be open or bivalve and constituted by different refractory materials (stone, sand, ceramic)

Cementite                      The hard and brittle compound of iron and carbon (Fe₃C, iron carbide) present in cast iron and steel where cementite is soluble.

Cold or hot working      The plastic deformation of a metal or of an alloy carried out a room or at high temperature (800-900°C) to permanently modify its shape. The cold shaping operation could include  hammering, bending and  rolling, flatten the grains and induces the presence of  strain lines and in ancient times was carried out to increase both strength and hardness.

                                        

Corrosion                       Corrosion is a gradual electrochemical degradation phenomenon of a metal or of an alloy due to its reaction with water and/or oxygen and agents.

Cuppelation                   The removal of lead from argentiferous lead by oxidation for the recovery of silver.

Dendritic structure       The dendritic structure is formed during casting and rapid solidification. Dendrites are crystals characterised by a tree-like branching pattern being most evident in cast metals slowly cooled through the solidification range.

                                       

Dendritic segregation   Not homogeneous distribution of alloying elements through the arms of dendrites.

Equiaxic grains                Polyhedral grains are crystals roughly equal in all dimensions, without acute angles and a more rounded shape.

                     

Etching                          Etching is a selective chemical corrosive attack with acids or other aggressive solutions to disclose the significant features of the artefact metallurgical structure to be used to identifying some technological aspects of the manufacturing process. Etching solutions are different as a function of the metallurgical features to be observed and are reported in literature. One of the most used for bronze is the alcoholic ferric chloride [5g (FeCl₃) + 10ml (conc. HCl) + 100ml (C₂H₅OH)] that is used to evidence the grain structure.

Eutectic                          In binary alloys, eutectic is the composition with the lowest melting point.

Eutectoid                       The eutectoid transformation occurs during the casting of  an alloy. The eutectoid transformation results in a separation of a single phase solid solution into two different ones. In a Cu-Sn alloy (bronze) the eutectoid transformation forms a layered structure that consists of the a and d phases and appears as a distinctive inter-granular constituent.

                                        

Firing                              A process of heat treatment carried out in a furnace under an air or a controlled atmosphere carried out to thermally treat an artefact or to transform clay in ceramic thus causing tailored and definitive structural and chemical-physical changes as well as to produce glossy decorative layers on the ceramic body.

Forging                          Heat and mechanical treatment carried out at high temperature (600-1000°C) where the metal or an alloy becomes malleable and it is possible to shape it by compression or exertion of force via hammering.

Gilding                          Gilding is the processes and the art of applying an Au leaf or thin film onto the surface of less precious substrates like metals, ceramics, marbles and wood with an artistic intent or for producing forgeries.

                                                   

Grain boundary             Bounding surface between the crystals. Grain boundaries are the preferred location for the impurities segregation phenomena or for the location of inclusions or not soluble metals with low melting temperature.                             

Grain size                      The average grain diameter expressed in millimeters at a magnification of 75x. Grain size of ancient alloys is controlled by the temperature of the thermal treatment and can be influenced by the duration and temperature of the thermal treatment (annealing).

                      

Granular fracture        An irregular surface produced when a metal or an alloy is fractured and whose appearance is granular or crystalline. It can be sub classified into trans-granular and inter-granular forms.

                               

Hardness                       Hardness is distinguishing property of a metal or of an alloy. Hardness test (Vickers, Brinnel, Rockwell) is used to quantifying a materials' ability to resist plastic deformation when force is applied from a standard source. The metal hardness is usually measured by indentation tests.

Interdendritic corrosion  Interdendritic corrosion is a selective preferential attack and penetration of corroding agents along the dendrites. The interdendritic areas are filled with corrosion products with different chemical composition and nature.

                                                                                         

Intergranular corrosion  Intergranular corrosion is a selective preferential attack and penetration of corroding agents along the grain boundaries. The intergranular areas are filled with corrosion products with different chemical composition and nature.

                                                                                      

Inverse segregation        During the solidification of a bronze and under specific cooling parameters, tin, arsenic or antimony can be forced to the interdendritic areas and to the surface of the object through the inter-dendritic feeders thus forming phases with a silver-like appearance.

                                      

Patina                               Patina is a layer grown on metallic artefacts and composed in many cases by different stratified corrosion products formed on metal surface from the exposure of the object to the atmospheric or soil components. Among the patina constituents could be: cuprite (Cu₂O), tenorite (CuO), malachite (Cu₂(OH)₂CO₃), azzurrite Cu₃(OH)₂CO₃), calcocite (Cu₂S), bornite (Cu₅FeS₄),covellite (CuS), nantokite (CuCl), brochantite (Cu₄(OH)₆(SO)₄), calcantite (CuSO₄5H₂O), smithsonite (ZnCO₃), calcopirite (CuFeS₂), leadlhillite (PbSO₄2PbCO₃Pb(OH)₂), cerussite (PbCO₃), piromorfite ((PbCl)Pb₄(PO₄)₃), romarkite (SnO) and cassiterite (SnO₂).

                                

Pearlite                            The eutectoid lamellar aggregate of ferrite (Fe) and cementite (Fe₃C) present in steel and case iron.

Polyhedral grains           Polyhedral grains are crystals having an acute angle. On the contray equiaxed grains are crystals roughly equal in all dimensions, without acute angles and a more rounded shape.

                                 

Re-crystallisation           Re-crystallisation is a phenomenon occurring at a precise temperature and for precise duration able to induce the    crystallisation of the deformed grains in a cold-worked artefact.

Salt Spray Test               An accelerated corrosion test. According to standardised methods metal specimens are exposed to a fine mixture of salt water solution either continuously or intermittently at a given temperature.

Segregation                     In an alloy, the segregation is the concentration of alloying elements or impurities at specific regions such as grain boundary surfaces, usually as a result of a heat treatment or during the cooling and the primary crystallization of one phase with the subsequent concentration of other elements or of the impurities in the remaining liquid. Segregation causes the not uniform distribution of alloying elements, impurities or phases and could induce embrittlement and inter-granular selective corrosion.

Shrinkage Cavity            Voids left in cast metals or alloys resulting from the volume change and solidification shrinkage. The voids could be filled by metals or impurities with low melting temperature.

                                     

Smelting                         Smelting is the process used to produce a metal from its ores via reduction reaction mainly carried out under a carbon monoxide atmosphere at high temperature in a furnace

Tarnishing                      Tarnishing is a layer of corrosion that is formed on the surface of metals or alloys. Well-known is the tarnishing of silver and its alloys. It is mainly caused by degrading agents present in the atmosphere such as sulphur. It appears as a gray or black film over the metal.

Thermal treatment         A process of heat treatment carried out in air or in a controlled atmosphere for example in order to soft a metal or an alloy and to shape or anneal it as well as to induce structural and chemical-physical changes in a ceramic or metallic material.

Tin pest                           “Tin pest” is an allotropic transformation of tin which causes degradation and brittleness of tin objects at low temperatures. Tin pest has also been called “tin leprosy” or “tin disease”.

Tinning                           The process for coating a copper or iron object with a thin layer of tin. The tinning could be carried out by dipping the object into molten tin or by heating tin and melting it on the surface of the object allowing to the molten tin to flow on the surface thus forming tin-enriched phases with the base metal.              

Twinning                         Twinning is the structure rearrangement in a metal artefact subjected to the combined effect of cold working and re-crystallization. Twinning induces the formation of typical features evidenced by parallel-sided forms within a grain.