Copper-bearing ore being broken apart (source: De Re Metallica, pg 272).
Since the beginning of intaglio printing in the 15th Century, copper has been among the most popular materials to hold an intaglio image. Artists like Martin Shoernjourer and Albrecht Dürer engraved their finest works on copper, while 16th Century artists Joseph Callott and Rembrant explored the new technique of etching using copper matrices.
While we now have access to a variety of suitable materials - plastic, aluminum, zinc, tin, brass, copper and steel - our choice involves an inherent compromise: too soft and the material will not hold up during printing, too hard and the plate will be difficult to work. During the height of long-run commercial intaglio engraving in the 18th and 19th Centuries, steel was chosen over copper for its durability. Hand working these steel plates was more difficult, and impressions are sometimes considered cold and detached, having been meticulously planned to avoid corrections. Softer materials like plastic and aluminum are less expensive and easily worked, but cannot stand more than a few printings before showing signs of wear.
With its favorable workability and longevity, copper has become the natural choice for professional and amateur printmakers. Despite its popularity, there is little specific information available to the modern printmaker on its composition and mechanical properties. This article will look briefly at the history of the copper matrix, the mining, refinement and processing of copper, and how its microstructure and mechanical properties affect its use as a matrix.
In his 1645 trieste, Abraham Bosse describes the best copper as ‘cuivre rouge’ - a pure variety for the time, since elemental copper gives it its reddish color. Bosse claims that yellow copper is ‘trop aigre & souvent pailleux & mal net’ (Bosse 12 ) - referring to the brittler and coarser brass alloy, letton. While red copper is less brittle, it can also be very soft. He cautions against using too soft a copper since the etchant of the time (nitric acid) acts slower on the softer metal, undercutting and thereby losing the delicacy of the lines being etched. In addition to its color, the printer should also source copper that is free of pits, inclusions or other inconsistencies. Testing it with a burin, Bosse suggests, one can quickly determine its suitability for both drypoint and etching.
Bosse goes on to discuss how to form the copper into a polished intaglio plate. It is first cold hammered into the desired size and thickness (as thick as a ‘teston’ - a 16th-century French silver coin for a 9” x 12” plate). The dents from hammering are removed by rubbing the plate with sandstone lubricated with water, followed by pumice and fine charcoal. The plate is brought to a mirror polish with a metal burnisher lubricated with olive oil.
It wasn’t until the introduction of rolling mills in the mid-19th Century that economy pushed printmakers away from hammered plates. In Treatise On Etching, Maxime Lalanne laments this shift, claiming the hammered variety was harder, less porous and capable of finer detail when etched (Lalanne 10). Andrew Raftery’s recent exploration of hammered copper plates lends a contemporary voice to the claim of their superior workability (Raftery and Campbell). By the mid-20th Century, however, hammered plates had all but disappeared. Printmakers like Ruth Leaf, Gabor Peterdi, and Stanley William Hayter began recommending the use of cold-rolled copper in its place.
Copper Processing & Milling
At the time Bosse published his treatise, copper mining and processing had a long history. Significant production took place in the Upper Harz (Germany), Cornwall (England), Falu (Sweden) and Rosos (Norway), along with numerous smaller mines throughout the continent. By the 17th-Century, copper had become a global commodity with Japanese sources sold into European markets. Given its widespread production and its diverse and labor-intensive processing techniques, it is not surprising that quality varied significantly during Bosse’s time.
Porphyry Copper Ore (source)
Most of the current global supply of copper comes from sulphide mines in Chile and Peru. Processing begins by crushing the mined rocks into a powder and separating the copper sulphide by means of flotation. This concentrated ore is sent for initial smelting to extract the iron, sulphur and non-metallic rock and further refined to a 99% purity. Additional purification (to 99.9%) can be achieved by electro-refining. Such copper is sold to mills as Electrolytic Tough Pitch (ETP) copper and given the UNS (Unified Numbering System for Metals and Alloys) designation C11000. ETP is the most common copper on the market and has a broad range of uses ranging from roofing to electrical switches and distillery kettles. Further commercial refinement can reduce its oxygen content resulting in exceptionally high electrical conductivity and low coefficient of expansion (UNS C10100, 99.99% purity).
Once purified, milling begins by casting the copper (along with any alloying metals) into a slab or ingot several inches thick. The slab is reduced to approximately ½” thickness by hot rolled - a process that elongates and refines the copper’s crystalline structure (i.e. grain). It is this grain structure that gives copper much of its important mechanical properties. The copper is then air-cooled to room temperature, and the black oxide formed during hot working is removed by surface grinding.
Slabs at Reveer Copper waiting to be rolled into sheets (source).
Further thickness reduction (to hundreds of an inch or less) is achieved during the cold rolling stage. Like hammering, this reduction process introduces dislocations into the copper’s grain structure, which buildup into ‘pinning points’ within the crystalline lattice. These points prevent further dislocations from moving through the structure when under stress - giving cold-rolled copper its hardness (temper).
Cold rolling uses the mechanical pressure of drawing the metal between two (or more) rollers to reduce its thickness (source).
Cold working, however, can only proceed so far before it becomes too brittle. If further reduction is necessary, the copper is softened (annealed) by heating it above its recrystallization temperature (typically ⅓ of copper’s melting point of 1085°C) for a specific period of time. During the initial stage of heating, dislocations migrate to grain boundaries, restoring the material’s ductility. Further annealing causes recrystallization - a process that initially refines the grains but, given time, can lead to grain growth. Cold-rolled sheet products of the type we are interested, are initially rolled to a ‘ready-to-finish’ thickness, annealed, and then cold-rolled to a final thickness where the annealing and further reduction creates the required grain size and temper. Commercially prepared cold rolled copper sheets conforming to ASTM B370 come in six standardized tempers: soft annealed (O60), ⅛ hard (H00), ¼ hard (H01), ½ hard (H02), ¾ hard (H03) and hard (H04).
In addition to grain size and temper, rolling also affects grain shape. With elongation occurring in the direction of rolling, grains appear closer together perpendicular to this elongation than they do parallel to it. Since tensile and yield strength are functions of grain size, they too will exhibit this anisotropic character. While grain deformation occurs in hammering, they do not line up in one direction as with rolling. Modern milling can reduce this directionality by ‘cross-rolling’ at 45° on each pass through the mill and by careful annealing to regrow equiaxed grains.
After cold rolling and annealing, copper sheets may be surface finished with a linear grainer and polisher ranging from 100, 200, 300 grit finish up to a mirror polished.
Important Characteristics for The Intaglio Printmaker
In classic drypoint, burrs are an essential part of the look and feel of the prints. If the copper is too brittle, the burrs will break off while working or when exposed to the pressure of the press; too ductile, and they will be too easily deformed in printing. The right balance is particularly important for mezzotint where repeated rocking is used to raise ink-retaining burrs. Ductility is also crucial during reworking since the artist needs to smooth out the old part of the image and work the area again without the copper becoming too brittle.
(ii) Yield Strength
Yield strength (the force required to make a mark on the plate) must balance the ease of hand working with durability during long print runs. A high yield strength also reduces the abrasive nature of the ink pigment from dulling highly polish areas, giving the artist more tonal control.
(iii) Grain Size & Shape
The finer the grain structure of the copper, the stronger the plate, the smoother the surface, and the faster an etchant will work on it. As Bosse points out, coppers requiring a long etch time will result in undercutting and the roughing of delicate lines.
Elongated grains resulting from unidirectional reduction rolling creates a plate with directionality (anisotropic strength characteristics) - a burin or drypoint scribe will tend to run more easily in the direction of grain elongation, making precise control of the tool more difficult for the artist. This characteristic can affect open bite and light etched tints (eg. lavis, where the tonality relies solely on the grain structure), causing a visible linear pattern running parallel with the elongation. In this case, an isotropic plate would be preferable: either hammered or cross-rolled with appropriate heat treatment.
Grain structure of ETP C110 hot rolled copper (left) and soft annealed copper (right); scale 50 microns (source)
(iv) Heat Resistance
When applying an etching or aquatint ground, printmakers expose their plates to elevated temperatures. Aquatint rosin melts at 100-120 °C, while grounds are often applied at lower temperatures. Although commercially pure copper does not show signs of softening below 180 °C, the cooking process should be done with care: hotplates are known to have hot spots and poor temperature control, while propane torches are capable of quickly heating the copper to well above its recrystallization temperature. Alloying copper with a small amount of silver (0.1% in the case of C11600) can increase the annealing temperature by 100°C without affecting the copper’s mechanical properties.
Plates need to be thick enough not to deform while being worked - a function of both plate size and its yield strength. They need to be thick enough to overcome any slight pressure differences over the roller’s width - since all but new or refurbished press beds and blankets will have some dipping in the center. Thin plates can also curl under the pressure of the press or warp when exposed to heat. On the other hand, plates that are too thick can cause the printing paper to tear at the plate edges or create distractingly wide platemarks in the final print.
To give the artist full control over their image, the plate’s surface should be flat, pit-free, smooth and capable of holding a high degree of polish. Unless specifically indicated, most suppliers will provide sheets with a ‘mill’ finish that produces a significant plate tone. Some specialty suppliers will sell ‘mirror polished’ plates that yield little or no tone. Unless the finish is protected with a removable plastic coating, any finish will likely show additional nicks and scratches from handling through the supply chain.
Mirror polished plate (left) and mill-finished plate (right)
Selecting Copper Plates
Copper alloyed with other metals will exhibit an increased rate of work hardening in proportion to the quantity of alloyed metal. Since ductility is necessary for drypoint techniques and reworking, pure copper is often chosen over alloys that will brittle quickly.
Although commercially pure copper comes in many grades, not all are readily available in sheet form, and even fewer are available in local metal supply shops. Oxygen-free electronic copper (OFE, C10100) and electrolytic tough pitch copper (ETP, C11000) tend to be the most common. The extra purity of OFE comes at a cost premium with no practical performance difference for the printmaker. For this reason, most copper sold to printmakers is ETP.
For ETP copper sheets, ductility, yield strength and grain characteristics mentioned in the previous section are interrelated and are specified in the trade by temper. Printmaking retailers do not indicate the temper of the plates they sell, and from personal experience, they seem to vary. Comparing them to known samples, I estimate most are of the softer variety (O60 or H00) and are preferable for drypoint applications. Etching and other techniques would not suffer significantly from these softer tempers, particularly if edition sizes are not large. Harder plates (H01 and H02) may have some benefits for etchings, aquatints and line engravings: crisper, stronger impressions over longer editions, finer details and retention of a higher polish. H03 and H04 are less commonly available and may be too brittle to allow for burrs or significant reworking.
The main market for C11000 sheet copper is roofing and architectural accents, with H00 being the most commonly used temper in that industry. While cross-rolling this type of copper is possible, there does not appear to be a need in the architectural market. In my experience, high-quality sheets that fit within the roofing specification (ASTM B370) can exhibit only a small amount of anisotropic characteristics. Some specialty metal suppliers will offer a range of finishing on these sheets, including mirror polishing.
Silver bearing coppers with their higher annealing temperature (such as C11600) are not readily available through local suppliers but are specifically manufactured for the graphics arts industry and sold to printmakers as ‘engraver’s copper.’ They are priced at a premium and are advertised as cross-rolled, thermally flattened, polished and come with an acid-resistant coating on the back. They are primarily sold to make printing dies for fine letterpress products. I have found these plates come with fine surface pitting that requires significant work to remove. The copper is very ductile (possibly soft annealed?) and has a ‘gummy’ feel when engraved. It does not hold a high polish and is susceptible to unwanted plate tone. To its credit, however, I have not found any anisotropic characteristics.
Most printing plates are sold in 16 gauge (0.0647”, 1.651mm) or 18 gauge (0.0485”, 1.245mm) thicknesses. Thinner than 18 gauge and the larger sized plates tend to bow excessively during handling and can develop bent corners. Thin plates with lower yield strengths also have a tendency to warp during repeated printings or when heated. The added strength of plates thicker than 16 gauge is generally not needed and only adds additional cost.
Having evaluated several copper samples from printmaking retailers, as well as specialty and local metal suppliers, there appears to be significant variation in milling quality. Of two samples purchased from one supplier, one plate had approximately 600 pits per square inch, requiring significant sanding to remove, while the other sample was almost flawless and needed only minimal work to bring to a full mirror polish. For this reason, Bosse’s advice seems to still ring true for contemporary printmakers: always check its suitability before you buy.
My professional preference is 16 gauge, C11000-H00, pre-polished plates. When collaborating with artists, the pre-polishing saves time while maintaining a wider tonal range and cleaner colors in multi-plate images. Additionally, pre-polished plates are generally more consistent in quality than unpolished ones. The ⅛ hard temper offers a good balance between ductility and longevity for the range of techniques artists may use.
In my engraving practice, I’ve recently begun testing ½ hard (H02) plates. The added hardness allows for finer details, cleaner cuts and a wider tonal range. Since I generally remove burrs from my engravings, I have yet to run into any disadvantages.
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