Resin, Pitch, Sap

by Holly Schubert

While walking in the forests at Lutherlyn and other places, we have occasionally noticed a sticky substance oozing out of trees, usually evergreen trees. Maybe you've seen it too, and if you've played in pine or spruce trees as a kid, maybe you've gotten this sticky goop on your hands, clothes, or even in your hair (yuck). I've wondered, and maybe you have too: what exactly is that stuff? (And for me, when I find something new in nature, another question that usually follows closely behind that is "can I eat it?" Or in general "how can I use it?")

Sap 

Many people assume that this sticky substance is sap, because it oozes out of trees. It is not sap. Sap is made up mostly of water, and carries water and minerals from the roots to the leaves through the sapwood (xylem) of a tree and carries water, sugar, and other nutrients from the leaves to the rest of the tree through the inner bark (phloem). Sap "runs" in sapwood when temperatures vary between above freezing during the day and below freezing at night. The temperature variation causes a difference in pressure inside the plant tissues, which pushes the sap up the sapwood during the day, and allows it to drop back to the roots at night. (This is when we can gather maple sap to produce syrup.)    

Resin 

The sticky stuff we see on evergreen trees (and some other plants) is not sap, or even hardened or thickened sap. It is a different substance altogether, called resin. The US Forest Service tells us that "resins are plant products that, 

• are not soluble in water, 
• harden when exposed to air, 
• do not play a role in the fundamental processes of the plant, and 
• are generally produced by woody plants.  

buck rub and resin flow on young norway spruce tree,
Venango Trail at Camp Lutherlyn

Resins are produced in special resin cells in plants, and are also produced when an injury occurs to the plant. Resins can be produced through the bark of a tree, the flowers of an herb, or the buds of a shrub." 

While resins do not "play a role in the fundamental processes of the plant" and are usually considered a waste product of cellular processes in the plant, they do provide a number of benefits to the plants that produce them. Resins seal over wounds, decrease water loss, protect from pests, and have antimicrobial properties that help prevent decay and infections.

Resins have also proven to be useful to humans. People have gathered and used resins from plants for thousands of years. Resins have been used for waterproofing, varnishes, adhesives, art, incense, medicines, food, and many other purposes. Some famous resins include  frankincense, myrrh, balm of Gilead, amber, and balsam. Some products that have been made from resins include pine tar, pitch, rosin, and turpentine, though some of these are now made synthetically or from petroleum products instead of plant resins.  

Amber is a fossilized resin. Though it is not a mineral, it has an appearance similar to a gemstone, and is often used for jewelry. Amber is created when resin hardens and then becomes trapped in layers of soil, which blocks off the oxygen that would cause it to decay. As the volatile oils in the resin slowly permeate out over millions of years, the resin becomes preserved, and turns to amber. Ambers have been found that are from 30 to 125 million years old! Occasionally, plant and animal matter that was trapped in the resin that became amber is also preserved, inside the amber. These inclusions can be used to extract DNA and study the flora and fauna of the distant past (as in the famous plot premise of the movie Jurassic Park). The resins that produced amber came from tree species that no longer exist today, but are most closely related to today's conifers, including pines.

All of the conifers common in Pennsylvania produce resins. Of the trees growing in our area, spruces, Scots pine, and larch have historically been used to produce products like turpentine from resin. At Lutherly, Norway spruce seems to produce the most resin, followed by white pine, and then other evergreens. In the northern woodlands that cover the upper reaches of the northeastern United States, balsam fir produces the most renowned resin. The resin is present in blister-like pockets that cover the bark, and its high oil content makes it simultaneously flammable, edible, and medicinal (though I imagine you might have to develop a taste for the flavor).

Some animals even use resin. One of the substances honey bees create in their hives is propolis. This sticky "bee-glue" is used to fill small cracks in the hive, smooth surfaces, and even seal off sources of contamination in the hive and keep the hive healthy. It is made from resins gathered from plants such as poplars and conifers, and mixed with saliva and wax. Propolis has also been used by humans for everything from embalming to antibiotics.  

Rosin is another product made from resin, used in fields from music to sports to industry. Musicians add it to the bow hairs of stringed musical instruments like violins; dancers, gymnasts, rock climbers and baseball players add it to shoes and hands to improve grip; and many industrial processes use it to create the right consistency in products ranging from chewing gum to lead soldering.  

Pitch 

People sometimes seem to use the word pitch to refer to the same substance as resin - that sticky stuff coming out of the trees. (I grew up calling it "pine pitch.") Or, in common usage it sometimes refers to the more fluid state of this substance, while resin is assumed to have hardened. But more properly, pitch refers to a product made from resin. Heating resin and mixing in a fiber and possibly a wax creates pitch, which is useful as an adhesive, waterproofing layer, or flammable material for firestarter, torches and crude candles. This is the type of pitch that native Americans used to waterproof the seams of birchbark canoes. 

Pitch can be produced from wood by heating the wood without burning it, which causes the pitch and tar to ooze out of the wood and leaves behind charcoal. (Pitch and tar are similar substances, but pitch is thicker and tar is more fluid.) Pitch was traditionally used to waterproof the seams of wooden sailing vessels, buckets, and barrels. The biggest producers of this type of pitch in the 1600's through early 1900's were Scandinavian countries and the North American colonies that later became the United States. 
At the same time, pitch is also a more general term for a number of "viscoelastic solid polymers." This means that even though it seems to be solid at room temperature and can be shattered with a hard impact, it is actually fluid and will flow over time, but extremely slowly. In this general sense, pitch can be natural or manufactured, from petroleum products or plants.

Naturally occurring asphalt or bitumen, a viscous semi-solid form of petroleum, is a type of pitch that has been used in "pitch drop experiments" to study and demonstrate the flow of a seemingly solid material over a period of many years. The University of Queensland in Australia and Trinity College in Ireland have both observed pitch over many decades, recording only nine  "drips" of the pitch in the course of their observations. In 2013 scientists at Trinity College were able to record pitch dripping from a funnel on camera for the first time. 

We still say something is "pitch-black" if it is very dark, because whether it is made from trees or petroleum, pitch is very dark in color.

 Gummosis

We have also noticed blobs of a gummy substance on wild black cherry trees, as well as cultivated cherry and peach trees. It is often mostly clear, or a little bit amber in color, and feels rubbery but not as sticky as the resin found on evergreens. I was surprised to discover that unlike the resin on evergreens, this IS a result of sap flowing out of trees. It is fairly common in "stone fruit" trees like cherries and peaches, and is referred to as gummosis. Sap from the tree oozes out of wounds in the bark caused by insects, fungus, or broken branches, and as it dries creates the gummosis. It is sometimes an indication that the tree is unhealthy because it is being attacked by insects or disease, but is sometimes simply the result of cracks in the bark or branches caused by wind or other damage.

It is amazing that the simple sticky blobs we see on trees are connected to so much history and fascinating information! For more reading about some of the topics mentioned, check out the links below. 

US Forest Service on resin

http://www.fs.fed.us/wildflowers/ethnobotany/resins.shtml

Temperate Climate Permaculture on resin (including biology, history and human use)

http://tcpermaculture.com/site/2014/08/04/tapping-the-pine-tree-plant-resins-and-their-uses/

Balsam fir, and resins in general

http://northernwoodlands.org/articles/article/blisters

Creation of amber from resin:

https://www.amberjewelry.com/Amber-Frozen-Moments-in-Time-s/97.htm

http://academic.emporia.edu/abersusa/whatis.htm

Propolis

http://basicbeekeeping.blogspot.com/2011/12/lesson-113-sticky-subject-of-propolis.html

Pitch

https://en.wikipedia.org/wiki/Pitch_(resin)

Historical production of pitch

http://www.maritime.org/conf/conf-kaye-tar.htm

Gummosis

http://www.missouribotanicalgarden.org/gardens-gardening/your-garden/help-for-the-home-gardener/advice-tips-resources/pests-and-problems/diseases/cankers/gummosis-of-fruit-trees.aspx