In the 1960s and 70s Knowles established himself as a major artist, but in 1973, just as his future as a major modern British abstract artist seemed assured, his uninsured studio caught fire in mysterious circumstances. It cost him most of his finished work, his work in progress and all his materials. What Knowles called "the silent time" began. After his 1973 show at Waddington Galleries, it would be 24 years before he exhibited again. His re-emergence as a public artist in the early 1990s showed that he had lost none of his power and inventiveness.
Although finished works in their own right, his prints can also be thought of as working models for larger paintings or three-dimensional objects. Knowles overriding concerns were with form, dimension and space, and he realised his works through the application of various systems to a set of ideas, what he called Ďartform conceptsí. Knowles often used the same concept, expressed in a combination of forms, to make works in different materials and on different scales, sometimes making work specific to a particular site.
Despite a large gap in his career (a studio fire in 1973 destroyed much of his work, and Knowles did not exhibit again until 1997), his work in the later period of his life shows a remarkable consistency to his early works. Knowles worked exclusively on non-figurative abstractions. In both his two-dimensional and sculptural work, he arranged or configured geometrical shapes (circles, triangles, etc.) and blocks of colour to explore phenomena such as symmetrical modularity, repetition and the use of space. From early in his career, Knowles developed his two-dimensional paintings into meticulously arranged sculptural forms, utilising materials like resin or fibreglass. For Knowles, the principal form of his work was the idea or concept, to which the materials used to realise the idea (whether canvas and paint, enamel, concrete, glass, steel, or marble, etc.) had a simply functional purpose.
Despite the apparently systematic nature of his working methods, his rigorous repetition and unrelenting use of geometry, Knowles acknowledges that his primary interest is in the forms themselves. As he says: ĎIím subjective, Iím not rational. People have often asked me whether Iím into mathematics, if Iím interested in systematics and stuff like that. And Iím not... My mindís totally visual.í
- Quotations Tate & the artist's obituary