Red Ochre Acts As A Hardener in Spruce Resin Pitch

The reddish brown glue under the
sinew, where the stone meets the
wood, is red ochre and spruce pitch

While I was in Alberta for the Archaeological Society of Alberta flintknapping workshops in Calgary and Edmonton, I experimented with spruce resin and red ochre glue.  I've used spruce resin and charcoal as pitch in the past, but I wanted to try mixing it with ochre because that seems to have been the glue of choice on the emaculately preserved darts and arrows that have been found in the Yukon ice patches.  My previous experiments with red ochre pitch made use of commercially prepared pine pitch, not spruce resin that I collected myself.

Sap oozes out of spruce
trees  wherever they've
been cut or damaged

I collected the spruce resin from a walking trail between St. John's and Cape Spear.  Spruce trees will bleed sap if they are wounded.  This can happen naturally from storms tearing off branches or even lightening strikes.  Groomed hiking trails or parks are good places to look for spruce resin, because the trunks will ooze sap wherever a branch has been cut off.  Collecting the resin while its cold out makes the work a little less sticky.  I use a sharp knife to cut the bigger globs off the tree and scrape the thinner layers of sap into a plastic bag.  I'm not exactly sure, but I think that the biggest clumps of resin were on the cuts that were a year or two old.

If you collect it on a cold day, its not very sticky and you can use a sharp knife to chip the gobs of resin into a bag.

Mixing the ochre.  The
 bits of bark that we
picked out of the melted
resin dot the paper beside
 the hot plate.

There is a lot of bark and lichen attached to the resin, but I don't worry about sorting that out until I melt the sap.  In the woods, you can melt the resin on a flat rock over a fire.  Be careful, its flammable.  In fact, you can use the spruce resin to help get a fire going while camping.  In the workshops, we melted the resin in a small frying pan on a hotplate.  You definitely want to use a dedicated frying pan, because it will be nearly impossible to clean it up afterwards.  The same is true for the hotplate - you can melt this stuff on your stove at home, but be prepared for some intense cleaning afterwards.

You don't want to boil the resin, so keep a careful eye on the pan.  Boiling the sap for too long will change it and it will become crystalized, rather than consistent and gooey.  While the resin was soft and runny, we picked out the bigger bits of bark and debris that were stuck in it.  Unlike a lot of the stuff that I boil, spruce resin smells great.  It smells like Christmas.

We ground the ochre into a fine powder with a mortar and pestle.

spooning in the red ochre

We ground the red ochre up with a mortar and pestle to make a fine powder.  When it was ground and the resin was melted and picked clean of debris, we slowly added the ochre to the glue and stirred it in.  We didn't measure exactly how much ochre, but I estimate that the ochre:resin ratio was somewhere between 1:2 and 1:1.  I don't think you'd want to mix in more ochre than resin, but that's just me.  Equal parts resin and ochre, or a little more resin than ochre seemed to work well.

Adding sinew and hide glue
over the pitch.

The ochre acts as a hardener in the glue.  Spruce resin at room temperature is soft and gooey.  Its very sticky, but it will stay pliable.  By adding charcoal, or in this case red ochre, the resin will be pliable at high temperatures but solidifies quickly as it cools.  The spruce resin and red ochre glue sets extremely quickly.  You have less than a minute to work with it before it sets and becomes solid.  Most of that time the glue is burning hot to the touch, but as soon as it becomes bearable to handle you can shape it and smooth it like putty with your fingers.  Again, its very hot and will stick to your fingers, so be careful.

Ice Patch artifact,
 the pink stain is
ochre and spruce
glue outlining the
shaft that it was
once hafted to.

Its best to plan your job carefully, because you have very little time to work with the glue before it sets.  I usually dab the glue into the wood socket and then jam the point into place so that the glue squishes out around the edges.  I pinch and tap down the excess pitch that oozes out to smooth the transition between the wood shaft and the stone point, to create a more aerodynamic shape that would penetrate the target more easily.  There are a couple good examples from the Yukon ice patches where you can see exactly this pattern; the ghostly silhouette of the wood shaft is visible and the glue was spread around the edges of the wood, over the surface of the stone point.

The pitch alone will do a pretty good job of securing the point in place, but its still a good idea to do a sinew wrap over the join and down the wood shaft.  The sinew will protect the pitch from chipping and will help prevent splitting in the wood shaft.  The sinew wrap also contributes to smoothing out the transition between the stone and the wood, which again, improves the aerodynamics of the projectile and helps the point penetrate deeper into the target.

Willow shaft, chert projectile point, red ochre and spruce pitch, sinew and hide glue hafting.  This is the foreshaft that I made in the workshops, using a little  Hoko Knife to work the wood 

Hafted point from
Calgary workshop.

We didn't experiment with the strength of the glue.  I recall from hearing Andrew Zipkin talk about his experiments with adding ochre to plant resin that in the best case scenario the ochre glue was just as strong as the ochre-free plant resin.  In most experiments it actually weakened the glue.  Which makes me think that ochre wasn't used to make a tighter bond, but it does change the properties of the pitch by acting as a hardener.  Spruce resin on its own is gooey and soft at room temperature, but once a bit of ochre is mixed in it changes.  For a few seconds, while it is cooling, it can be shaped like putty and it rapidly solidifies into a hard, water resistant glue that creates a very strong bond between stone and wood.


Photo Credits:
1-7, 9, 10: Tim Rast
8: Screen capture from The Frozen Past: The Yukon Ice Patches.