Such as seen in Chardin’s “The Soap Bubble”?
What I mean is, what color(s) did he use to demarcate the bubble? Is there a standard technique for this sort of thing?
The surface of the bubble will diffract light passing through it. So there will be some distortion in the appearance of the objects behind the bubble. Also any light sources in the area will be reflected off the bubble’s surface. And there’s an edge effect where light will be diffracted enough that it will appear as white light to a viewer, so the edge of a bubble can be painted as a thin white circle.
I don’t understand the OP. All you have to do is look at it, and you’ll know what colors were used. And how could there be a “standard technique” for painting bubbles? Is there a standard technique for painting faces? Or horses? Or landscapes? The greatest skill in painting is the ability to see.
Think of it as reflecting light more than image. Then take into account the angle of incidence, where there is more concentration of specularity on the edges that are closer to 90 degrees from your viewing angle.
The distortion of the relfection on a sphere is such that what is behind it is visible from in front - just squeezed into a thin line along the (apparent) circumference.
As people mentioned, what you are seeing when you look at a colorless, transparent object is the distortion of the objects behind it. This is because the bubble, glass, whatever has a different index of refraction than the air, or IOW light moves at a different speed.
So if you look at the painting, you can see that the edges are very light versions of the color behind the bubble. They are then blended with the background as you move towards the center of the bubble. The background behind the bubble is distorted “fisheye” style and a light color is used to paint some specular highlights (light source reflections). Note that pure white isn’t used as it would make the bubble contrast the background too much and look unnatural.
Well, really what you’re looking at isn’t distortion or refraction or anything-- it’s a 2-dimensional surface, and. . . well, similar to what Panache said-- look at it and you can answer your own question. It’s a bit of light-colored paint scumbled on gently, and the blue paint scumbled on, and some spots of white paint, to suggest what your eye thinks is highlight. Forget for a moment what’s supposed to be represented, and look at what’s there in paint.
Thank you. BTW I never got an email that anyone answered the OP. So I had to Search to find out.