Acrylic paint is a water-based fast-drying medium. But acrylic paint's fast drying time is both a blessing and a curse. It is convenient for the immediate storage of paintings, but can be problematic when blending acrylic paints together on a canvas. This affects the techniques for painting fire.
Appearance of Flames
Flames are usually whiter and brighter in a fire's centre, and dimmer and more red or orange toward the outer edge of the flame. The flame itself may take on any number of shapes, but most often the flame is wider at its base and pointed in one or many peaks at the top. When painting a flame, it's best to look at a photograph of a flame and draw the shape of the flame on the canvas before beginning the painting.
Painting in Layers
Flames are built up in layers. The layers of the flames should begin in the centre and radiate outward in a shape that mimics the centre, like layers of an onion or the rings inside a tree. Once the outside of the flame has been drawn, the interior layers may be drawn to match the outer layer. There should be no more than two or three layers. After the layers have been painted, use a dry brush to blend the layers together. Run the dry brush over the boundary between each layer.
Keeping the Paint Wet
During the painting process, blending the paint will be crucial to the development of a realistic flame. As the acrylic paint dries, it will become gummy, and blending the paint will become more difficult. To keep the paint wet, the paint may be blended with water before applying it to the canvas. Watered-down paint will need to be applied to the canvas in multiple layers. Alternatively, a water bottle may be kept on hand to spray the canvas with a fine mist. Acrylic paint may also be mixed with acrylic retarder to extend the drying time of the acrylic paint. Acrylic retarder may be purchased from art and craft stores that sell acrylic paint; follow the manufacturer's instructions.
Most artists find the most challenging part of painting flames to be recreating the colour of the flame. What the artist discovers is that actual flames are brighter than paint can realistically reproduce. In reality, flames are lit from within. In paintings, flames are not lit at all, they are just painted with bright colours. A mixture of white, yellow and a small amount of orange may be used to paint the inside of the flame. Use more yellow than white. For the exterior of the flame, more orange and a small amount of red may be used, with less yellow and white.