Oil paintings are 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 practicitioners 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 absorbant ground was perfected. Colors in oils were intially 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 colors.
The supports of choice for oil paintings are wooden panel or cloth (linen). Poplar was the wood of choice for Italians with walnut 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 a calf hide-based or fish-based glue. 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) is 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 is achieved this way. This type of mixing is referred to has grinding and yields a more consistant and well-distributed paint than mixing with a knife, spoon or other implement alone.
Gold leaf gilding was common on the religous 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 is completed. This can be either on parchment to be tranferred to the wooden panel, or a drawing directly on the panel itself.
- An ink wash is applied to fix the drawing to the panel. The ink wash is used to reinforce the drawing and acts as a guide to the painting moving forward. A stiff brush or feather is then used to remove all charcoal from the panel.
- A full value underpainting is applied over the ink drawing. The colors in this underpainting are very limited or monochromatic. This is called grisalle. In parts of Italy a verdaccio underpainting was used which uses the colors of black, green, and pink. The underpainting may egg tempera or in oil paint.
- Glazes of oil paint are then applied over the underpainting, adding color to the previously muted or monochromatic underpainting. Glazes are applied in layers as each previous layers dry. Some peroid paintings have upwards of twenty layers of color. Glazing is a technique developed in oil painting. It allows the full strength of a color to be applied without tinting it white, or shading it with black, both of which dimish the color strength of a color.
For painting on cloth:
- A charcoal drawing is completed. This can be either on parchment to be tranferred to the cloth, or a drawing directly on the prepared cloth itself.
- A full value underpainting is applied the drawing. The colors in this underpainting are very limited or monochromatic. This is called grisalle. In parts of Italy a verdaccio underpainting was used which uses the colors of black, green, and pink. The underpainting may only be in oil paint because the ground used for cloth is an oil ground.
- Glazes of oil paint are then applied over the underpainting, adding color to the previously muted or monochromatic underpainting. Glazes are applied in layers as each previous layers dry as noted above for paining on wooden panel.