Mr Barks is feared dead along with fellow workers Jason Shingler, 38, Derek Moore and Dorothy Bailey, both 62.
The families of those missing will visit the site – owned by Wood Treatment Ltd, part of The Boden Group of Companies – later today.
A joint investigation is under way between police, the fire service and the Health and Safety Executive to establish the cause of the blast.
Assistant Chief Constable Guy Hindle said: “We continue to work with our colleagues in very difficult conditions with ongoing risks- it is an extremely complex scene to examine and we’ve not yet got full access to all of it.
“Each agency has a wide range of expertise and we are working hard together to find the missing people.
“In the meantime we are interviewing witnesses and gathering other evidence, all of which will form the investigation.”
Temperatures reached 1,000C during the explosion and blaze, which caused the collapse of the four-storey building.
The site was also struck by fires in 2010 and 2012.
Wood is processed at the plant into a fine powder and the resulting “wood flour”, with a consistency like sand, is used to make laminate flooring.