Offset Printing
Offset printing, an indirect printing method, is named after early lithography, which initially used stone plates. In early lithography, stone blocks were ground flat and used as plates. Later, this method evolved to use metal plates made of zinc or aluminum, but the principle remained the same.
In offset printing, there is no height difference between the printing and non-printing areas; they are both flat. It utilizes the principle of the immiscibility of water and oil to maintain a layer of oily film on the image area, while the non-image area of the plate can absorb water. The idea is that after inking the plate, the image area repels water and accepts ink, while the non-image area absorbs water and repels ink. This method of printing is called "offset printing."
Offset printing originated from the transfer method of early lithography, where images were transferred onto transfer paper and then onto the plate as a reversed image, which was then printed onto paper as the correct image. However, the pressure exerted during printing caused the ink to spread beyond the intended lines due to the originally flat nature of the plate, resulting in poor line quality. This led to improvements, known as the "offset printing process," where the plate surface is made as the correct image, transferred onto a rubber roller as a reversed image during printing, and then onto paper as the correct image, thus improving the elasticity of printing pressure.
Early offset printing was flat-bed or flat-bed rotary, later evolving into two types: flat-bed rotary and rotary rotary. Flat-bed rotary machines were mostly used for special printing purposes such as proofing machines, while rotary rotary machines were used for printing on paper and other materials. In flat-bed rotary printing, the printing plate is placed flat, and the pressure is applied by cylindrical rollers. This method is similar to flat-bed rotary machines in letterpress printing. In rotary rotary printing, the printing plate is wrapped around a cylinder called the plate cylinder, while another cylinder on the machine, wrapped with rubber, called the blanket cylinder, applies pressure. Machines using these three basic cylinder configurations are called "offset printing presses."
Advantages of offset printing include easy plate-making process, low cost, accurate color registration, ease of plate duplication, smooth and soft color tones, and the ability to handle large print runs.
Disadvantages include reduced color fidelity and vibrancy due to the influence of water and ink, thin ink coverage on the plate (only utilizing about 70% of its capability, requiring double-sided printing for posters to enhance color), and limited applications for special printing needs.
In 1798, the Austrian (now Czech in Prague) composer Alois Senefelder (1771-1834) invented lithography, a printing method that directly prints on a flat surface, thus initiating the technique known as planography. He also developed a wooden lithographic press. Senefelder initially studied law but later pursued composing, although with little success. Facing financial difficulties, he turned to printing his compositions for sale, but printing costs were high and profits meager. He then apprenticed at a printing house but lacked the capital to innovate on his own. While living near the Solnhofen Quarry near Munich, where there was an abundance of limestone, used for grinding copper plates or as a substitute for copper plate engraving, he observed his mother washing clothes and accidentally wrote on a limestone slab with greasy ink, which he found could not be washed away after several days. This led him to experiment with greasy ink on limestone, first writing text in reverse on the stone, then applying a mixture of nitric acid and gum arabic followed by ink, which ultimately succeeded. He referred to this as "chemical printing." The term "lithography" derives from the Greek words "stone" (litho) and "writing" (graphein), now synonymous with planography.
In 1834, Senefelder became the Inspector of the Bavarian Royal Print Works, but he passed away in February of the same year.
In 1810, a German, F. Weishaept, assisted Senefelder in completing an iron hand-operated lithographic press.
In 1817, Senefelder attempted to use zinc plates instead of limestone for lithography, but without success.
In 1826, a Frenchman, Niepce, invented the heliographic process.
In 1832 (Qing Dynasty, 11th year of Daoguang), the British missionary W.H. Medhurst established a lithographic press in Macau for printing Chinese books.
In 1837, a Frenchman, G. Engelmann, invented chromolithography.