This agate arrowhead / scraper (?) was found on a saltwater beach (Puget Sound, Washington). You can see that it is covered with a white patina.
My question is how to date arrowheads/tools such as this: How much time does it take to develop a white patina like this example? Obviously the actual chemical composition and the minerals it has been exposed to play a big part, but what does that mean time-wise? Anyone have any information / knowledge as to the aging process in order to rough guess age?
Also, I've noticed that this type of white patina seems to happen primarily to agates, and the rest of the beach stones do not have a patina like this...
There have been attempts to measure it (using a similar process to obsidian hydration) but I think it ended up being too inconsistant to measure that way (and probably not the same thing across different rock types.) You see that frosting in Agates, Agatized Corals, Knife River Flint, high grade Edwards, and a couple of other materials most commonly. On some materials the hazing goes away with a long soak in water or mineral oil, in others it seems to be permanent.
Personally, I don't think it takes as long as many people think. I have a site I hunt in the desert and I've found points exposed that all have been sun bleached like that on one side. I've also found points when I dug into the undisturbed deposit that didn't have it. The disturbed area was bulldozed in the 60's to flatten out an area for an oil well set up. So in less than 40 years one side of the agate points picked up the patina.
I've been collecting agates for a long time, with some of my 'yard rocks' being exposed to the elements for at least 40 some years now. Some have taken on a green tinge from our rainy & cool Northwest weather, but not one has turned white with patina. That's why I was so surprised to see so much of the white patina on these 'new' artifacts.... must be a difference in minerals and the saltwater?
Here's photo of a flake from another Puget Sound agate with heavy white patina, with a small window into the agate. No way to guess age by it's shape or design. It's interesting that the 'newly exposed' interior portion of these agates is what typically has the most patina rather than the more age exposed outer rind.
I also grew up on Puget Sound. Didn't see an arrowhead for 60 years, nor knew anyone that had found one - until I found the one above 2 years ago. Got a chuckle about being a hero for finding a a Puget Sound point... you obviously know how rare they are.
So you can understand my amazement that after reading your post, I went out again and found this.. I actually did feel like a hero when I picked it up !
The rate that patina forms depends on many things like soil ph, moisture, how deeply buried the material, etc. Under the right conditions cherts and agates can patinate in as little as 500 years, but usually it takes about 1500-2000 years to see pronounced patination. Again, patina can not be used to determine age, but if an artifact is patinated it usually means that it's at least 1500 years old (in the soils around here that is).
A common misconception about patina is that it is caused from sun bleaching. Sunlight may play a role in some types of patina formation, but really, how long can an artifact remain exposed before it gets covered up by grass, soil, etc.? Lots of artifacts are patinated on one side, the side that faced up. The upward-facing side patinates faster than the other side because water collects on that side, while under the ground..
I think patina is an interesting topic. The problem, though, is that different areas produce different patinas and "patina" is a term that is thrown around for different things like creek staining, caliche buildup, etc.
Also, patina isn't a coating, it's the material actually changing color.
Twitch, Welcome to the obsession !! Patina is a funny thing. It affects differ based on basic chemical make-up and light. I've found edwards grey flint out in the desert that's been on top the ground for milenia that looks as bleached as a coke bottle on one side and retains some grey on the other. By the same token I've pulled stuff out of six foot deep holes in aluvial bottem land ,that shows that they too have laid in the sun for some time before being covered up by nature. Ground chemistry too plays a part in all this. Lone Star
The rate that patina forms depends on many things like soil ph, moisture, how deeply buried the material, etc. Under the right conditions cherts and agates can patinate in as little as 500 years, but usually it takes about 1500-2000 years to see pronounced patination. Again, patina can not be used to determine age, but if an artifact is patinated it usually means that it's at least 1500 years old (in the soils around here that is).
A common misconception about patina is that it is caused from sun bleaching. Sunlight may play a role in some types of patina formation, but really, how long can an artifact remain exposed before it gets covered up by grass, soil, etc.? Lots of artifacts are patinated on one side, the side that faced up. The upward-facing side patinates faster than the other side because water collects on that side, while under the ground..
I think patina is an interesting topic. The problem, though, is that different areas produce different patinas and "patina" is a term that is thrown around for different things like creek staining, caliche buildup, etc.
Also, patina isn't a coating, it's the material actually changing color.
It is interesting how people define patina.
One of the many areas I hunt points is a high desert plateau just north of the equator. High deserts can the some of the highest levels of uv radiation in the world, daily temp swings of 80+ degrees, little rain but consistent morning dew, and very little wind to move dust or sand around.
Desert varnish is probably the best understood forms of patina, here is a brief description. In some places it can happen in as little as 50 years.
Basically there are two major processes involved in patination, selective leaching and matrix dissolution or desilicification. Selective leaching occurs when elements such as iron, aluminum in addition to quartz are deposited in solution and transferred from the interior of the rock to the surface where they are redeposited in higher concentrations. This usually happens in acidic environments.
Matrix dissolution(desilicification) is a surface weathering process and represents the mass wasting of silica from the surface of the stone. This patination is controlled by two factors pH and temperature. Some studies have shown that patination intensity does increase with age but there are so many variables it can not be reliably counted upon.
Yes, patina can refer to just about any change in appearance and there are lots of things that can cause it. The type of patina (dissolution of silica) that I'm referring to is common on Knife River flint and other cherts and chalcedonies and occurs underground usually.
You can often tell how an artifact was positioned in the soil by how it's patinated. Areas on the artifact where water tends to accumulate are the areas that will show the most patina. I think it's cool when the upward-facing side patinates and the patina wraps around the edge but stops soon after it gets past the edge. Also, heavily ground/polished edges will often show heavy patina like this Cody piece. I think water tends to accumulate and stick to the ground edges? Any thoughts on this?
This piece shows the patina that occurs commonly on just the one side on KRF and other cherts.
This knife is only about 500 years old, but it already has pronounced patination on this side.
So it is like what happens to clear bottles that are exposed to sun? I see it in creeks and rivers but figure that is from mineralization. Thanks, I learn something new every time I check in here
LinkBack URL
About LinkBacks
Reply With Quote
Originally Posted by coteau
