Complex sentence: It was a reaction against a decline in standards that the reformers associated with machinery and factory production, sharpened by items in the Great Exhibition of 1851 that were thought to be ornate, artificial and ignorant of the qualities of the materials used.
Split into simpler sentences: It was a reaction against a perceived decline in standards that the reformers associated with machinery and factory production. Their critique was sharpened by the items they saw in the Great Exhibition of 1851, which they thought to be ornate, artificial and ignorant of the qualities of the materials used.