Friday, March 22, 2013

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.


  1. Have you ever experimented with birch pitch?


    1. Not yet - only pine and spruce so far. Any tips? A lot of my reproductions are based on artifacts found above the treeline, so I don't often get an excuse to use pitch.

    2. There is several online sources that describe birch tar/pitch production as well as some youtube demos ( I haven't tryed it myself, no birch trees in Churchill). What I found interesting about it is that it needs no additional material added to it to be hard at room temp. It seems to be a natural plastic. Given the large amounts of birch in the province birch pitch could have been a viable resin.


    3. From what I gather, you burn the bark to extract the tar, so wouldn't charcoal and ash be part of the mix from the beginning? Its added in one step, but I think there is still more in birch tar than just tree sap to help with the hardening.

    4. From what I have seen of the process it is so what like the production of char cloth in that the birchbark is roasted in a chamber with a very low oxgen level. The oil weeps from the bark and runs down into a collector container beneth (sometimes in a water bath to cool the container). Then the oil is reduced by cooking into a tar. There may be some carbon introduced but I don't think there is much charcoal or ash in the mix.

  2. This is really interesting, and got me wondering if you have come across any traditional/native uses for frankgum? Or is chewing frankgum just a simple, useless, pleasure? :)


    1. I had to look up frankgum and I think we might be talking about the same stuff. If its Spruce Gum, then its the same pleasant smelling goo that runs down spruce trees. You can chew it and it also makes a good glue. Pitch was used as an adhesive on many Innu tools in Labrador, but I'm not certain of the species of trees that were used as a source for the sap. Spruce, Pine, and Birch are all possibilities. On the Island, the Beothuk were recorded as using pitch made from spruce gum, charcoal and fat as a sealant on their birch bark canoes.

  3. Hi Tim,
    Andrew Zipkin here; I'm really excited to see that you're also working on experimental reconstruction of ochre and spruce resin adhesives. My conclusions only apply to adhesives based on gum arabic (Acacia senegal resin). Acacia resin takes a while to dry at room temp, at least a few hours and really to be safe, a whole day. But when it dries it tends to be very brittle. Adding some sort of loading agent (ochre, sand) makes it qualitatively less brittle. My tests look at how much force an adhesive bond can stand up to when loaded in tension (pulling apart two thing glued together at an overlapping surface). Acacia resin without anything added to it makes a really good adhesive; adding an ochre loading agent can actually make the adhesive weaker but perhaps that was an acceptable trade off if the ochre also made the glue less brittle.

    Where did you get your ochre from and what size were the particles? Fine sand, silt, clay? Particle size seems to be an important determinant of adhesive strength in my most recent study:

    1. Hey Andrew, I'm glad you caught this post. Hearing you talk about your experiments definitely influenced my thinking on adding ochre to pitch/resin. It seemed from your work that if ochre has a function in the mixture, its not to increase the strength of the bond, but something else.

      The ochre I used was collected in Newfoundland from a community called Ochre Pit Cove that is an hour or two drive outside St. John's. It seems like a hematite based ochre, but I haven't confirmed that with a geologist. Its some sort of iron rich rocky outcrop. At the outcrop, I collect bags of ochre that range in size from small pebbles to silt size particles which I then filter or grind as I need it. The ochre that we added to the spruce resin would have been in the silt to clay size range after grinding.

      Adding the ochre to the resin, did make a better glue than spruce resin alone, in the sense that it turned something with the consistency of thick honey into something solid like epoxy at room temperature. But as you've illustrated in your experiments - I don't think ochre is unique in that property. I think you got similar results with quartz and other additives. Adding *something* to plant mastic seems important to make a better adhesive, but outside of the colour, I'm not sure why ochre would make it into the top ten list of minerals to mix in.

      I'm not sure if you make it back to St. John's very often, but I'd be happy to take you out to Ochre Pit Cove to check out the ochre source for yourself.

  4. I'll let you know if I'm back working at MUN again sometime next fall after my field season in Zambia this summer. I've actually never seen an ochre source outside of Africa.

    One possibility to consider is that your spruce resin plus ochre adhesive is like an epoxy in a literal sense; it's possible that the ochre is acting as a hardening agent by catalyzing some chemical reaction. Quartz is pretty much chemically inert so any effect it has on the gum arabic in my experiment would likely be due to purely mechanical processes. But iron-rich ochre might be changing the pH or some other property of the resin and causing it to dehydrate or polymerize. You would need to find some literature on the chemistry of spruce resin.

    1. The epoxy analogy is a good one. As you say, it would take more research to understand if the chemistry is comparable, but the end result is very much like mixing an ultrafast setting epoxy.

  5. Hey, my names Luke Webb and I live on Deer Island N.B. I run a small business making somewhat similar things to what you do and sell local farmers markets. I am really a big fan of this blog, checking it regularly. Quite recently I have been making up spruce pitch and just collected a nice load of it on a walk in an improvised birch bark container that I will be making into pitch for hafting knives. I have always used charcoal and deer dung in it, I have found the charcoal to actually soften the pitch rather than make it harder, but I could be wrong. The deer dung being from an ungulate is all fiber so by powdering that and adding it in it acts like reinforced concrete re-bar and makes the pitch much stronger. Another softening agent I have read can be used is fat, but I have never gotten around to trying it.
    In my experience I like a slightly harder pitch for hafting knives as it holds the blade a little firmer than the softer pitch which can eventually loosen over time from the blade being pushed side to side in warmer weather and it can be exposed to sun and heat longer without softening and the blade going crooked. However, for anything that is recieving a shock like an arrow or spearshaft a hard pitch will shatter, so soft pitch is vital.
    One other trick that I have tried is adding sand, by adding sand you make a very heavy pitch and rather than use it for hafting it makes ideal fishing weights, you can even form the gooey pitch/sand around your line, hook, plug or flasher hot and it will be on permanent, or you can put a hole through it while it is still soft and make a removable sinker.
    The final thing I've learned is that if you want to work with the pitch, mold it and such, while it is still to hot to the touch is to simply lick your fingers. If you lick your finger you can mold the sticky hot pitch without it sticking to your finger and smooth it all out really nice.

    1. Hey Luke, thanks for all the great recipes and tips. I remember you from Palaeoplanet.

      I think the way the Ice Patch hunters worked around using a hard, inflexible ochre and spruce pitch on their projectile points was to add a healthy sinew wrapping over the glue. The arrows and darts with sinew lashing still preserved would minimally have the shaft wrapped to prevent splitting, but the sinew frequently covered the entire glued area as well. That's going to help hold the point in place, but its also going to help prevent the stiff pitch from flexing and shattering. Its kind of like adding a protective coating of fibre-glass over the brittle core. Some of those points have been held in place for several thousand years, so a hard pitch of spruce gum and ochre, mustn't be too bad to use on some projectile points.

      I guess I really need to do some side-by-side comparisons of ochre and charcoal additives in spruce gum. The ochre pitch definitely dries hard, but it has a density to it as well that makes it feel a little more robust and less brittle than charcoal pitch.

      I haven't tried adding grease to the mix yet, but the Beothuk were supposed to use spruce gum, charcoal, and bear grease to make a flexible pitch for waterproofing their canoes. The Beothuk loved ochre, and ochre was used as a hafting additive in lots of places and times around the world, but I don't know of any evidence that the Beothuk used ochre in their pitch. Its a little odd.

    2. I was out making pitch all day yesterday, trying different recipes. I tried the addition of fat and found that it did soften it a little bit but also changed the texture and strength, it wasn't as strong and had a texture kind of like plaster if that makes any sense, for example if you scratched it with your fingernail it would all crumble rather than shatter like hard pitch or scrape off in gooey manner like soft pitch, it just kind of crumbles like plaster.
      I always have a really hard time getting my pitch soft, I have never used anything but spruce gum as that's what I have growing on Deer Island, so I wonder if that doesn't have something to do with it. I succeeded in getting one batch to come out soft the other day, the trick was to use FRESH WHITE sap from a wound and about 1/3-1/2 dried rosin picked from the side of the tree. This came out soft even without adding anything to it, I then added charcoal which made it much more flexible and pliable though not softer (acts as a binder I've now found,) and deer dung really upped the strength.
      When adding fat be very careful not to add to much, you only can use a small amount otherwise just like wax it separates and forms a coating of it over the pitch when it dries and doesn't seem to change the consistency of the pitch any.

      I've never been down to Newfoundland, do you have many birch trees? I have always wanted to try making birch bark tar, do you know if there is a way to do this without using clay pots? I have heard it is a wonderful mastic adhesive but it requires quite a lot of apparatus to make so you really can't make it on the go if you were an indian.

  6. you could try mixing beeswax along with the fat to act as emulsifier


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