(Not a teacher)
Here is a slightly more concise way of saying this:
In the afternoon of the day this work began, De Lesseps, who was speaking happily with Kale, ripped the protective cloth from (a, the) valuable Rubens and suddenly shouted in dismay.
If the Rubens was one of a number of articles (paintings, etc.) and was not specified, use "a" as the article. If the Rubens had been previously specified, use "the."
Student or Learner