The evolution of 19th-century photomechanical printing relied heavily on the selection of metallic substrates capable of sustaining high-pressure press runs while maintaining microscopic tonal fidelity. Copper and zinc emerged as the primary materials for photogravure and etching plates, each offering distinct mechanical properties and chemical sensitivities. While copper became the standard for high-quality artistic reproduction due to its ductility and fine grain, industrial-grade zinc provided a cost-effective alternative for the mass production of periodicals and commercial catalogs.
The structural integrity of these plates was subjected to significant stress during the 'biting' process, where acid etched the image into the metal, and subsequent printing cycles, where mechanical friction and atmospheric pollutants accelerated degradation. In the industrial urban environments of the Victorian era, specifically in cities like London and Manchester, archives of these plates were exposed to high concentrations of coal smoke and moisture. This exposure triggered complex chemical reactions that altered the surface topography of the metal, often compromising the latent image's clarity and the plate's long-term archival utility.
At a glance
- Copper (Cu):Valued for its high ductility and tensile strength (approx. 210 MPa), allowing for deep, detailed etching and long-run durability via steelfacing.
- Zinc (Zn):Utilized for its lower cost and hardness (approx. 140 MPa), though it is more susceptible to brittle fracture and granular corrosion.
- Biting Agents:Ferric chloride was the preferred etchant for copper, whereas nitric acid was frequently used for zinc, resulting in different microscopic edge profiles.
- Atmospheric Pollutants:Sulfur dioxide (SO2) and particulate matter from coal combustion were the primary drivers of plate pitting and sulfidation.
- Archival Medium:Lignin-free cotton rag paper served as the primary substrate for transfer, requiring specific alkaline buffering to counteract acid migration from deteriorating plates.
Background
The transition from manual engraving to photomechanical reproduction in the mid-1800s required metals that could be sensitized with light-sensitive bichromated gelatin. When exposed to light, the gelatin hardened, creating a resist that allowed acid to bite into the metal in proportion to the image's tonal values. This process, perfected by figures like William Henry Fox Talbot and later refined by Karl Klič, demanded a substrate with a uniform crystalline structure. Any impurities in the metal would cause uneven etching, leading to 'foul biting' or artifacts in the final print.
Copper was traditionally preferred by engravers because its atomic structure allows for a smooth, predictable reaction with ferric chloride. Zinc, while easier to obtain in large industrial sheets, possesses a larger grain structure that can lead to a coarser texture in the highlights of a photogravure. As the printing industry expanded in the late 19th century, the choice between zinc and copper became a balance between the desired aesthetic quality of the image and the economic requirements of high-volume publishing.
Mechanical Properties and Tensile Strength
The physical demands of the printing press required plates to withstand repeated cycles of compression and friction. During the intaglio printing process, the plate is inked, wiped, and then pressed against dampened paper under several tons of pressure. Copper exhibits superior tensile strength and ductility compared to zinc, which is vital for preventing the plate from warping or cracking over time.
| Property | Industrial Copper (1890s) | Industrial Zinc (1890s) |
|---|---|---|
| Tensile Strength | 200–220 MPa | 130–150 MPa |
| Hardness (Brinell) | 35–45 | 30–40 |
| Melting Point | 1,085°C | 419°C |
| Corrosion Resistance | High (Stable Patina) | Moderate (Prone to Pitting) |
Zinc is inherently more brittle than copper. In high-speed industrial presses, zinc plates were more prone to developing micro-fractures, particularly around the edges of the etched areas where the metal was thinnest. This brittleness also made zinc less suitable for 're-entering' or manual corrections by an engraver, as the metal would chip rather than displace smoothly under a burin.
The Biting Process and Micro-Topography
The 'biting' process is the chemical etching of the image into the metal plate. In photogravure, a screen or a grain of bitumen is used to create a network of cells that hold the ink. The depth and width of these cells determine the tonal range of the finished print. Copper reacts with ferric chloride in an exothermic but controlled manner, allowing for a vertical bite that preserves the sharpness of the grain.
Zinc, however, reacts more violently with nitric acid, which was the standard etchant for the metal in the 19th century. This reaction often resulted in lateral etching, or 'undercutting,' where the acid eats away the metal beneath the protective resist. This phenomenon compromises the structural integrity of the cell walls. Over the course of a long press run, these weakened walls can collapse, leading to a loss of detail and a 'muddying' of the middle tones. Researchers analyzing historical plates have noted that copper plates maintained their micro-topography significantly longer than zinc plates, even when subjected to similar levels of mechanical wear.
Impact of Victorian Industrial Pollution
Archives located in 19th-century industrial centers were not climate-controlled environments. The pervasive use of coal for heating and industrial power released massive quantities of sulfur dioxide into the atmosphere. When combined with atmospheric moisture, this formed dilute sulfuric acid, which is highly corrosive to both copper and zinc.
Deterioration Patterns in Zinc
Zinc plates from this era frequently show signs of 'white rust' or zinc carbonate/hydroxide formations. In the presence of coal smoke, zinc reacts to form zinc sulfate, which is water-soluble. This process leads to deep pitting of the plate surface. For a photomechanical plate, even microscopic pitting is catastrophic, as the pits hold ink and appear as dark spots or 'noise' in the highlights of the printed image. Furthermore, the brittle nature of zinc meant that these pits often served as initiation sites for larger cracks during handling.
Deterioration Patterns in Copper
Copper is more resistant to atmospheric corrosion but is not immune. In Victorian archives, copper plates often developed a dark brown or black patina of copper sulfide (Cu2S) or copper oxide (CuO). While this patina can sometimes act as a protective layer, the presence of soot and particulate matter often trapped moisture against the plate surface, leading to localized 'bronze disease' or chloride-induced corrosion. This would result in the expansion of the metal surface, distorting the etched image and making the plate unusable for further printing without extensive restoration.
Mechanical Wear and Image Fidelity
The lifespan of a photomechanical plate is measured by the number of high-quality impressions it can produce before the friction of the wiping process wears down the etched highlights. Wiping involves a printer using a stiff muslin cloth (tarlatan) to remove excess ink from the surface, leaving ink only in the etched recesses. This constant abrasion eventually polishes the plate, reducing the depth of the cells.
- Steelfacing:To combat wear, copper plates were often 'steelfaced' using an electroplating process that deposited a thin layer of iron over the surface. This allowed for thousands of impressions. Zinc, being more reactive, was more difficult to plate effectively, often resulting in poor adhesion of the protective layer.
- Friction Degradation:Zinc’s lower resistance to abrasion meant that without plating, the delicate 'aquatint' grain of a photogravure would begin to fade after as few as 50 to 100 impressions.
- Pressure Distortion:The pressure of the rollers can cause 'elongation' of the metal. Zinc plates showed a higher rate of dimensional distortion than copper, which could lead to registration issues if multiple plates were used for a single color image.
Archival Inscription and Cellulose Substrates
The final stage of the photomechanical process is the transfer of the image onto a cellulose substrate, typically paper. The material science of the paper is as critical as the metal plate for the preservation of the visual narrative. Historically, high-quality prints used 'rag paper' made from linen or cotton fibers, which are naturally low in lignin. Lignin, found in wood-pulp paper, breaks down into acidic components that cause yellowing and brittleness.
"The longevity of the recorded image is a function of the chemical harmony between the residual ink, the metallic ions transferred from the plate, and the alkaline buffer of the cellulose substrate."
When a plate is printed, trace amounts of metal ions can be transferred to the paper. If the paper is acidic, these ions can catalyze the degradation of the cellulose fibers. Modern conservation techniques emphasize the use of alkaline buffering agents, such as calcium carbonate, to neutralize these acids and prevent chromogenic degradation. This is particularly important for historical prints where the 'biting' process may have left residual etchants or metallic salts within the ink layers, which can migrate into the paper over decades of storage.
What sources disagree on
There is ongoing debate among technical art historians regarding the efficacy of various historical 'varnishes' used to protect plates in storage. Some period manuals suggest coating plates in beeswax or tallow to prevent corrosion, while others argue that these organic fats can turn rancid and produce fatty acids that further etch the metal surface. Additionally, modern spectroscopic analysis has challenged the long-held belief that all 'zincographs' of the 19th century were pure zinc; many were actually complex alloys containing lead or cadmium, which significantly alters their corrosion profiles and the toxicity risks for contemporary conservators.
