Two defence witnesses in the Robert William Pickton murder trial have raised some doubts about bloodstain evidence introduced earlier in the trial by Crown prosecutors.

Gordon Ashby, an expert witness in the "identification of unknown substances through applied chemistry," told the trial in the B.C. Supreme Court in New Westminster on Monday he believes some of the stains aren't blood at all, but different types of glue.

He tested bloodstains on a mattress found in a motorhome on Pickton's Port Coquitlam property. The Crown had alleged the mattress contained some of Mona Wilson's blood.

Wilson was one of the six women Pickton is on trial for killing between 1997 and 2002. The other five are Andrea Joesbury, Sereena Abotsway, Georgina Papin, Brenda Wolfe and Marnie Frey.

Pickton awaits trial on 20 more counts of murder. All of the charges stem from the disappearances and deaths of women from Vancouver's Downtown Eastside. He has pleaded not guilty to all charges.

During Ashby's testimony, the crown questioned his academic background, noting he only had a diploma from a technical school. It also got him to agree his laboratory is not certified for forensic testing.

Robert Pickton is on trial for the deaths of six women who went missing from Vancouver's Downtown Eastside.
(Jane Wolsak/Canadian Press)

But Ashby was granted the right to testify as an expert because of a long history identifying unknown substances using infrared technology.

He tested two of the stains from a mattress found in the motorhome, using a process involving infrared light. He testified that the resulting spectra resembled various types of glues, not human blood.

He testified that many types of glue are made from animals, and the infrared signature he saw in his lab suggested a type of protein used to make adhesives.

Another forensic specialist from Washington State, who helped to recover human remains at the World Trade Center site after the Sept. 11 attacks, also doubted the bloodstains were human.

Jon Nordby told the court he analyzed the mattress stain by performing an experiment using his own blood and piece of foam.

Nordby testified the experimental blood pattern differed in appearance from the stain on the Pickton mattress, suggesting the substance on the mattress wasn't blood.