This is an interesting question! I will endeavor to answer your question to the best of my ability. First, a bit of history.
Chafing dishes were popular in England during the turn of the 20th century. They were used to cook mostly delicate dishes such as eggs, oysters, fish, and cheese. Here's a picture of an English chafing dish circa 1870.
Edith Wharton's The Custom Of The Country was first published in 1913. Here are some chafing dish recipes to give you an idea of the kind of food that can be cooked in chafing dishes. Today, chafing dishes look like this.You will often see them in buffet lines, whether at a restaurant or at a wedding reception. There is a flame source beneath the chafing dish; the food is usually kept in a metal basin with a water bath separating the food section from the heat source. Food is kept warm in chafing dishes after it is cooked so that guests can help themselves to warm food whenever they are ready.
Now, to get back to our novel, let's first discuss Mr. Popple. He's the artist who has been commissioned to paint Undine Spragg's portrait. His personality as an artist is such that he is very careful to hide all the messy preparations that go into his work from his clients.
To sitters for whom this was of the first consequence it was another of the artist's merits that he always subordinated art to elegance, in life as well as in his portraits. The "messy" element of production was no more visible in his expensively screened and tapestried studio than its results were perceptible in his painting...
Laura Fairford refers to Mr. Popple's art as 'chafing-dish' art. Mr. Popple considers it the very epitome of good breeding in an artist to hide the occupational trials and struggles of his work from the eyes of his clients. He is the gentleman artist, if you will. No messy, intermediate evidences of his work must mar the image of the elegant connoisseur of beauty.
It was his opinion that the essence of good-breeding lay in tossing off a picture as easily as you lit a cigarette. Ralph Marvell had once said of him that when he began a portrait he always turned back his cuffs and said: "Ladies and gentlemen, you can see there's absolutely nothing here,"...
So, how does this relate to the chafing-dish art image? In wealthy English households, much of the preparation work would have been quietly done away from guests in the kitchen. The assembled ingredients would then have been allowed to cook slowly at table, eventually producing elegant chafing-dish suppers for the guests. So it is with Mr. Popple's art: all the messy art work is done away from prying eyes. What he allows his clients to see are the final finishing touches designed to give the impression that he produced the portraits effortlessly.
Hope this helps!