The most gracious source was a white pine under which we BBQ all the time! A copious amount of gum had accumulated under a branch wound and had already begun to dry out and turn grey. Using an old broken knife, I harvested the gum into a can and scraped off other sources
Gob of gum; Harvesting with an old knife
When I started harvesting from these sources, I would notice a large grub wiggling around in the bark. Further research has pointed out that I've been getting assistance from the Pitch Mass Borer (Synanthedon pini), a type of moth that deposits eggs on the bark of the host pines and spruces during June and July. The larvae bore in the inner bark and sap wood excavating tunnels and cause copious amounts of sap to flow as a defense mechanism. Usually a single larva is found in one glob and don't damage tree extensively.
Apparently removing the larva causes the wound to seal off completely and stop flowing so I've been careful to ensure that I don't kill the delicate larvae while also leaving some protective gum on the wound as well. Afterall, these little guys have made my harvesting sufficient quantities of gum much easier and the trees are supplying me with a fantastic waterproof sealant.
At anyrate, with all plenty of pines and spruce in the vicinity that have been trimmed for landscaping purposes, I've been able to collect a whopping load of gum. I ended up transferring all the collected stuff into a plastic bag and then stuffed it into a 1kg tub (formerly containing some fantastic Medjool Dates). After softening a bit in the summer sunshine, the 2 pounds or so of softened gum has taken on the shape of the container.
1 Kg of gum in the container
The unsightly mix needed to be filtered to remove bits of bark, soil, needles, and other debris. So I rigged up a cooking station with an Trangia alcohol stove, used an old cheapo pot to melt the gum and poured the resulting brew into an aluminum foil lined pan topped with a cheese cloth "filter". The pitch melted easy enough into a bubbling brew and occasional flared up like a flambe so doing this outdoors with a lid nearby to smother the pot was a must. After thoroughly boiling to cook off the bulk of the pitch smell (apparently making the gum less brittle), it was poured into the cheesecloth and the clean gum squeezed. This required some quick work because the cooling gum would solidify rapidly.
The set-up; Melting globs of gum
To reduce the extreme stickiness of the gum and limit the mess, I put the pan into the freezer for a while and that allowed the resin to break off the aluminum foil quite nicely. The chunks were then stored in a zip lock bag. After repeating this process a few times, I ended up with about 500 grams of purified resin (from the original 1kg amount). This isn't enough to seal the boat so I've been actively collecting more gum from any and trees I come across (eliciting strage looks from passers by).
Purifed batch of gum; Putting it in the freezer
Purifed Pine Resin; The total harvest
I also found that the cotton cheesecloth tended to trap too much liquid gum, so my filtered yield wasn't as much as I had hoped. Both Jim Miller's DVD and Cesar's Bark Canoe video show the gum filtered with a burlap bag whose woven fabric seems to allow much more liquid gum seepage. After searching for burlap bags in vain, I posted an ad on Toronto's Freecycle and Bill responded to me request with plenty of burlap bags. I intend to use this with the next batch of gum preparation and expect to get better results. Thanks Bill!