Is zakat obligatory on artworks that were painted for trade, given that only the blank canvas was purchased, and the painting was done through the artist's own effort and not through a paid exchange? And how is zakat calculated in this case?

If canvases are bought with the intention of selling them after painting on them, they are considered trade goods, and Zakat is obligatory on them, even if they are blank when purchased. An example of this is someone who buys land to build upon and sell; it is appraised when a year passes and Zakat is paid on it. However, if canvases are bought for painting on them without the intention of selling, and then one decides to sell them, they are not considered trade goods in that instance. If one sells them, a new lunar year begins for the price received, provided it reaches the Nisab (minimum threshold for Zakat).