Foam in Forest Firefighting
Barry Marsden (bio)
This paper sets out the history of Class A Foam use in Victoria, giving full details of equipment development and the advantages and disadvantages of the various approaches.
The nature of water makes its conservation difficult, since it evaporates, beads up and will not penetrate or stick to vertical surfaces. One method which assists with overcoming these difficulties is adding foam compound to the water.
In Victoria, from the mid 1950's until the early 1980's, water was treated with "wetting agent" to reduce the surface tension of the solution, and allow it to spread and penetrate fuels more effectively. Wetting agent plus water was known as "wet water". Wetting agent was available in powder form in cellophane packs from the mid-fifties, and as a liquid in more serviceable plastic packs from the mid-sixties. It was introduced directly into the water tank, then mixed in by vehicle motion or by pump impeller rotation. Although it was an advance over using plain water, "wet water" could not be made into foam. Also, pump priming problems due to cavitation were frequent. An improvement had to be found, and the answer lay in both equipment development and the introduction of Class A Foam.
Following reports of successful trials in Canada and France, in which Class A foams proved superior to wetting agents for wildfire suppression, foams were introduced into the forest fire operations in the early 1980's
What's so good about foam?
Foam is visible and sticky. The ability to see where it has been applied avoids over-treatment, and thinning areas pinpoint where the foam is breaking down, so identifying hot spots. Its stickiness enables foam to cling to vertical surfaces, providing an insulating blanket which reflects radiant heat and reduces waste through run off.
The above introduction is based on this article from 1995
and there is additional detail in this paper from 1996.