The painting itself is very large, reaching 200 cm by 162 cm, approximately. It is believed to have been purchased in 1997 by the Nagoya City Art Museum and joins a good selection of Mexican art to be found within this Japanese gallery. It is rare to find such items in the country, with European art much more common, alongside the best local artists. Proletarian Unity was completed on three layered plaster which would have made it a little difficult to then move on once sold, but the overall piece was moved on to board which works as the base and ensures the overall piece is more solid in its construction. Rivera used the techniques of fresco art to create this painting, many of which had been used for many centuries and were perfected during the Italian Renaissance. Rivera would work in many different mediums across his career, including oils for smaller pieces as well as some watercolour from time to time. Rivera regularly worked on series of murals and in this case, Proletarian Unity was a single part of a wider project known as Portrait of America.
Within the overall series he would focus on different events within the life of the USA, including the War of Independence and the Civil War. It was highly up to date, reaching events right up to the creation of the painting in 1933. Many will immediately identify some of the people included within this painting, such as Lenin, Marx, Engels, Stalin and Trotsky. One would immediately be confused as to why so many Russian Communists would appear within a series of paintings devoted to the history of the US, but one must look deeper into the history of this artwork to understand why. The painting was commissioned by America's socialist movement leader, Lovestone, and it was a call to arms to fellow Americans to join his left-leaning persuasion. The figures included were therefore people whom he wanted to feature and shape the country in the future, if his struggle proved successful. Rivera was entirely onboard with such ideals and so was happy to put this piece together.
The Japanese museum in Nagoya City holds some of the finest art from outside Japan anywhere in the country. Most galleries, as mentioned, concentrate on Japanese art plus a number of Impressionists and Renaissance artists from Europe, but the Nagoya City Art Museum looked further afield when constructing its own permanent display. Aside from Proletarian Unity, you may well also be interested in viewing the likes of Girl with Death Mask by Frida Kahlo, Mexican Landscape (Paisaije Mexicano) by José Clemente Orozco, Girl with Pigtails by Amedeo Modigliani and also Pilgrims by María Izquierdo. It is likely that they had direct connections with several Mexican collectors in order to acquire so many works from this region.