In what kind of art can mass be observed?
If you're asking what kind of art can be viewed by the masses, then you have to pick a time period. Prior to the Modern, or industrial age, art for the masses was typically limited to themes of religion and art which supported a State. Each culture had its own art for the masses and then there was high art, often associated with elitists or aristocrats or clergy. It depends on the time periods and in which parts of the world you're talking about with regards to things like: the degree of rigid definition between "high" art and mass art. Clearly, the lines of restriction, censorship and availability of art, especially based on class systems will vary depending no time and place. But in general, art has been practiced by all classes; from cave paintings to the Sistine Chapel; each to represent or suppose some idea, culture, reality or metaphysical concept. As Modernism developed, along with the age of mechanical reproduction of art (photograph, video, movie, etc.) art of all types from all periods of history became available to the masses. This movement in art continued the blurring of lines between concepts of high art and pop (or kitsch) art. Anything from cave painting to Monet can be seen in advertising on the internet set to Mozart or Rage Against the Machine. Everything now overlaps (pastiche). A more literal answer to your question is that art seen by the masses would be in public; museums that are free or art outside where it can't possibly be censored: from churches to installations to graffiti.
From my understanding, I think you are talking about mass as in the quantity of space within a form, in other words, what makes the object more three-dimensional. If so, then I suggest you looking at realistic paintings or drawings of real life objects or people because of the amount of shading/values being incorporated within the piece.
If this question is a riddle, the answer would is a church or cathedral. Architectural structures are artistic buildings that serve an aesthetic taste in addition to their function as shelter or a gathering space. Mass (the service) is held in cathedrals, which are art in an architectural sense.
I think that mass is just a concept to illustrate quantity and hence to put to art becomes an issue. In reality mass does not exist, its similar to speed, it is just hypothetical illustration of some quantity. However one could in reality illustrate a force(weight) due to an object that has mass. To put it in art, i would draw a picture of a person carrying a object, however dipicting a "human expression" of a person under either a heavy or light object. In that way you could illustrate mass in art. This "art" observation is ofcourse under the assumption that the "art" is at a point were the accelation due to gravity is constant over extreemly wider radias, that u could approximate that area of radius to infinity relative to the point of the "art".