This is my demo from today’s painting class, an acrylic study of glass bottles on cardboard (the back of a sketch pad). Cardboard makes an interesting surface for painting with acrylics. The medium brown tone of it is a nice starting point, and it takes the paint nicely, smooth surface/good absorbency. Of course, it isn’t archival, but for studies and quick casual sketches, it is a nice way to recycle the backs of sketch pads and other cardboard items.
Palette on this = pthalo blue, cobalt, alizarin, yellow ochre, cad yellow, red ochre, naples yellow, burnt umber and titanium white. A good start with some very dark darks via a burnt umber underpainting set the tone for sharp transparency and brightness later on. Don’t think of glass as being “light”, glass is whatever color/value is reflected in it or seen through it, as well as affected by the color of the glass itself. Transparent objects need dark areas in order to look transparent on paper or canvas. The darks are what make the brightest brights and highlights appear more luminous, without them, everything will lack dimension and look flat.
Keeping your colors pure is also important–establish the darks and then place the medium value colors next to them, resist the urge to over blend. Over blending will give you mud and reduce the dramatic contrast between darks and lights that are juxtaposed such as they often are in reflective surfaces. If you are working wet-into-wet in oils, this is critical. Acrylics are a bit more forgiving because they dry quickly, but you can still end up with a muddy mess if you attempt to over blend.
When you get to the last details and are ready to place your highlights, remember to use both warm and cool tones, and always add color if you are using white. Highlight are never pure white, they always pick up some warm or cool tones from the light source. I use yellows for warm highlights caused by incandescent lighting, blue violets for cool highlights from window light or flourescents. Using these various temperatures will give you a more natural looking highlight and make the color variation more interesting overall.
Painting glass is not difficult, the same principles of form and value apply to glass as they would to a non-reflective/non-transparent object. Glass is simply more complex, you have more sub-forms to look for when working out your values. Get the underpainting right, and the rest of it will come easily!