, but 
safflower, poppy, and walnut oils are used as well. The different oils have 
different properties in terms of how long the curing process takes, how hard the 
oil film becomes, how resistant to cracking or yellowing it becomes, and so 
forth.</p>
<p>Once cured, a painting needs relatively little in the way of protection, 
although varnish is common to protect against atmospheric pollutants. Still, 
over time, the painting may be susceptible to cracking and/or yellowing. These 
risks can be le