Oil paintings were executed using a paint consisting of a pigment in a drying oil (linseed oil, walnut oil, etc). Oil painting is believed to have been developed in the early 15th century in Europe and has enjoyed popularity since then. Wooden panel was the support of choice for early practitioners of oil painting with cloth supports becoming popular in Italy in the late 15th century. The paint films formed in dried oil paintings allow light to pass through the many layers of semi-transparent glazed passages of paint to create a luminous effect previously unachievable in any other painted medium.
The roots of oil painting lie in egg tempera painting where the technique of thinly layered paint over an absorbent ground was perfected. Colours in oils were initially used as accents to egg tempera paintings. Eventually artists began using oil paint to complete whole pictures. Many period oil paintings on panel have an underpainting in egg tempera which allowed the artist to perfect composition and tone before applying the final layers of rich oil paint colours.
The supports of choice for oil paintings were wooden panel or cloth (linen). Poplar was the wood of choice for Italians with oak being favored in northern Europe. Wooden panels that were aged, free of oils and knots were chosen. Cloth supports were linen canvas that were clean and free of oils and grease. Cloth was stretched tightly on wooden frames to provide the artist with a working surface less likely to bend and flex.
Period size and grounds
Both wooden and cloth supports were first prepared by sealing their surfaces with size, a glue usually calf hide-based or fish-based. A gesso ground (consisting of slaked plaster and glue) was then applied to wooden supports to provide a receptive working surface for oil paints. Because gesso dries to an inflexible and rigid state, it is unsuitable for use on cloth supports. An oil ground (lead white in a drying oil) was used instead for cloth supports.
Oil paint was created by mixing pigment with a drying oil. Pigment and oil was placed on a glass or stone slab and ground together with a glass muller which is flat on the bottom. The even dispersion of pigment within the oil was achieved this way. This type of mixing is referred to has grinding and yields a more consistent and well-distributed paint than mixing with a knife, spoon or other implement alone.
Gold leaf gilding was common on the religious paintings of the time. The gold leaf was sometimes applied in the early stages of a painting, but sometimes near the end. The glue used to adhere gold leaf to the painting was called mordant and has a reddish tone. After gold was applied to the mordant, the gold would then be burnished to a high sheen with a semi-precious hard stone or a hound's tooth.
For painting on wooden panel:
- A charcoal drawing was completed. This was either on parchment to be transferred to the wooden panel, or drawn directly on the panel itself.
- An ink wash was applied to fix the drawing to the panel. The ink wash was used to reinforce the drawing and acted as a guide to the painting moving forward. A stiff brush or feather was then used to remove all charcoal from the panel.
- A full value underpainting was applied over the ink drawing. The colours in this underpainting were very limited or monochromatic. This is called grisalle. In parts of Italy a verdaccio underpainting was used which uses the colours of black, green, and pink. The underpainting may in egg tempera or in oil paint.
- Glazes of oil paint were then applied over the underpainting, adding colour to the previously muted or monochromatic underpainting. Glazes were applied in layers as each previous layers dried. Some period paintings have upwards of twenty layers of colour. Glazing is a technique developed in oil painting. It allows the full strength of a colour to be applied without tinting it white, or shading it with black, both of which diminish the strength of a colour.
For painting on cloth:
- A charcoal drawing was completed. This was either on parchment to be transferred to the cloth, or drawn directly on the prepared cloth itself.
- A full value underpainting was applied over the drawing. The colours in this underpainting are very limited or monochromatic. This is called grisalle. In parts of Italy a verdaccio underpainting was used which uses the colours of black, green, and pink. The underpainting may only be in oil paint because the ground used for cloth is an oil ground. Also, egg tempera is not flexible when dried so it is unsuitable for a cloth support.
- Glazes of oil paint were then applied over the underpainting, adding colour to the previously muted or monochromatic underpainting. Glazes were applied in layers as each previous layers dried as noted above for paining on wooden panel.