The growing mastery of the principles and technique of painting accompanied a change in the painter's attitude toward his art. Originally, painting, applied in subjection to architecture and employed in the service of the Church, was decorative in scope; its purpose was illustration, its function was to teach. As painters, from generation to generation, went deeper into the secrets of their craft, they became less interested in the didactic import of their work, and they concerned themselves more and more with its purely artistic significance. Religious subjects were no longer used merely as symbols for the expression of piety and as incitements to devotion; they became inherently artistic motives, valued as they furnished the artist an opportunity for the exercise of his knowledge and skill and for the exhibition of lovely color and significant form. A change in the mechanical methods of painting, also, had its influence on a change in the conception of the function of art. With a very few exceptions, the works of Giotto were executed in fresco as wall decorations. The principles of mural painting require that the composition shall be subordinated to the architectural conditions of the space it is to fill and that the color shall be kept flat. The fresco method meets these requirements admirably, but because of its flatness it has its limitations. The introduction of an oil vehicle for the pigment material, in the fifteenth century, made possible a much greater range in gradated color, and reinforcing the increased knowledge of light and shade, aided in the evolution of decoration into the "easel picture," complete in itself. Released from its subjection to architecture, increasing its technical resources, and widening its interests in the matter of subject so as to include all life, painting becomes an independent and self-sufficing art.