It was an exhibition opening at Seattle's Greg Kucera gallery, and the artist whose works were on the walls was doing what artists do in these situations, accepting congratulations, chatting up visitors, being agreeable. Another artist (a sculptor as it turns out, but that hardly matters) approached him, wanting to talk about the paintings. However, the sculptor expressed displeasure about the imagery and called the edges "sloppy." The painter, who asked that his name not be mentioned, defused the situation by walking away. "The guy didn't have any sense of protocol or sensitivity to the situation," he said. What is the protocol?
Certainly, there are other opportunities to have serious discussions about artwork, with (at an artist's talk) or without (a review) the artist being present, while exhibition openings are events to honor the artists and, perhaps, make a sale or two. It's a party, and people are supposed to be on good behavior, keeping conversation light, not eating or drinking too much and not making a scene.
There is a lot of etiquette in the art world that, like most etiquette, tends to be understood implicitly rather than spelled out. Etiquette describes the expectations of social behavior, maintaining a shared zone of comfort for everyone involved. For instance, visitors tend to speak quietly (at least, about the art) in a gallery, in order not to intrude their thoughts about the art on the experience of other people there; conversations with the artist at an opening tend to be brief, so that the artist may meet and greet as many visitors as possible; discussions with the artist at an opening should eschew how much the art costs. Was anything sold? What does the artist make in a year? It's probably not great manners to ask about if there are any good restaurants in the neighborhood, although like the prices of the art and the fact of sales, the name and location of a good eatery is not unreasonable to think about.
Brief conversations with the dealer also make sense, talking about the artist, the artwork or the prices (appointments should be made for lengthier discussions), but exhibit openings are not good times for other artists to show their own artwork to the gallery director or to make a pitch for their own shows.
Speaking ill of other people generally is poor form and reflects badly on the speaker. Artist A who ridicules Artist B in front of collectors or dealers may adversely affect the fortunes of Artist B but, more likely, will make those collectors and dealers wary of having anything to do with Artist A, a person known to talk dirt behind someone's back. Being an artist, as the old saying goes, is an excuse to make art, not an excuse to behave badly.
Many artists look to break the supposed rules of art in their work, but the protocol of behavior at art events appears to be more fixed, in large measure because it is the same general code of conduct at other occasions. Everyone likes to hear (positive) feedback but far fewer want criticism, especially in person. Public criticism is itself a display, competing with the art in the gallery. Critical comments in a published review or posted online may engender anger, but the criticism is not as immediate (they may have been written hours or days or weeks before) and they are not as personal (the writer is not speaking directly to the artist). There is time and distance to cool down. The painter at the Seattle gallery opening had the presence of mind to just walk away from the rude sculptor, which may be the best response. If the offense had been inadvertent (the sculptor hadn't meant to irritate), the artist might have seemed overly sensitive, touchy, egocentric. Had he lost his temper, his outburst might have been the only thing visitors recalled from the opening.