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The Chemical Composition of various types of ash

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I've been thinking too much about ash lately. I hope it won't bore anyone, but I thought i'd share some of what I've found about the chemical composition of various types of ash. I hope you guys find some use for it--maybe in choosing charcoal types, or an appropriate ash for forge and smelter floors.. None of these numbers are hard and fast, varying from place to place and between each individual members of species. If the numbers don't add up to 100%, assume a loss on ignition.


All data is taken from Digitalfire Ceramic Materials Database(link: http://digitalfire.com/4sight/material/ )



Hardwood ash (specifically Oak):


CaO: 21.51%

MgO: 15.85%

K2O: 33.44%

Na2O: 2.30%

P2O5: 16.34%

SiO2: 0.67%

Fe2O3: 0.58%

MnO: 2.60%

SO3: 6.71


Softwood Ash (Pine):


CaO: 41.10%

MgO: 8.21%

K2O: 10.96%

Na2O: 0.77%

P2O5: 4.67%

SiO2: 2.00%

Fe2O3: 0.07%

MnO: 2.54%

SO3: 6.69

Loss on ignition: 23%


Applewood Ash


CaO: 54.14%

MgO: 4.20%

K2O: 9.01%

Na2O: 1.45%

P2O5: 3.43%

SiO2: 2.06%

LOI: 25.70




You'll notice that there's very little silica in any of these ash types, the bulk being lime. Cherry and Eucalyptus contain some silica:




Cherry Ash (water soluble portions washed away)


CaO: 41.00%

MgO: 12.00%

P2O5: 10.00%

SiO2: 33.50%

Fe2O3: 3.50%




Eucalyptus Ash


CaO: 20.26%

MgO: 11.94%

K2O: 10.33%

Na2O: 10.83%

P2O5: 3.41%

Al2O3: 2.21%

SiO2: 32.90%

Fe2O3: 3.50%

MnO: 0.51%

SO3: 4.11%



But rice husk and rice straw ashes are nearly pure silica. I think these ashes could serve very well in applications requiring refractories.



Rice Husk Ash


CaO: 0.49%

MgO: 0.22%

K2O: 0.91%

Na2O: 0.26%

P2O5: 0.01%

TiO2: 0.16%

Al2O3: 1.01%

SiO2: 96.70%

Fe2O3: 0.05%

MnO: 0.19%




Rice Straw Ash


CaO: 2.74%

MgO: 1.49%

K2O: 3.29%

Na2O: 0.56%

P2O5: 1.29%

Al2O3: 10.78%

SiO2: 77.26%

Fe2O3: 0.53%

MnO: 0.70%

SO3: 1.35%




Bamboo is also a source for nearly purely silica. I have been unable to find data of a similar kind for more commonly available straws (wheat, rye, oat, etc.), but they too are high in silica, though not as high as rice husk and straw ash.


I hope you guys get some use out of this information.

Edited by Tyler Miller
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That's good stuff, thanks Tyler.


What I find most interesting from a smelting point of view is the relative phosphorus levels between oak and pine.


I knew rice straw ash was mostly silica, which is why it's the traditional flux in Japan. I'd think most of the grasses are high in silica. I'd like to see an analysis of sawgrass ash, I bet it's very high silica indeed.

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Alan, I found it very surprising that the traditional Japanese flux, rice straw ash, is mostly silica and it doesn't seem to have much in the way of melters. I had in my mind a flux more like an cherry ash--something with more flux than glass former. I guess I didn't understand the process of fluxing iron as well as I thought. How Borax performs may have skewed my understanding and expecations.


I wasn't thinking about phosphorus levels when I put this up, I'll confess, but you raise an excellent point. Do you think if you did one smelt with oak, another with pine, you could get contrast in a blade made of both? Or did you have something else in mind?

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Thanks for posting the data..I have seen much higher and lower numbers for the K2O in the rice straw ash..the melter is the iron oxide of the steel..that is the beauty of the material.



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Silica is the traditional flux for wrought iron and bloom steel. Borax came in with modern steels that require a bit more oomph to remove the oxides due to the lower welding temperatures modern steels require compared to bloomery products. As Jan said, it's the iron oxide itself working on conjunction with the silica. Since wrought iron and bloomery steel have a fair amount of silica in the form of slag worked into them due to method of manufacture it only makes sense to add a little more if you need to flux a weld. If you look at old smithing book that predate the common use of homogenous modern steels you'll often see things like "clean white sand" or crumbled up mud dauber nests (they find natural stoneware clay, aka AlSiO, to build their nests from) mentioned as aids for difficult welds. Wrought often has enough iron silicate slag that it needs no flux at all.


And you guessed my train of thought exactly. Phosphoric iron. I do not know if iron smelted with a high-phosphorus charcoal will absorb enough to make a difference, though. A few years ago some of us were thinking about adding bone meal or phosphate rock to a smelt to see what would happen, but IIRC somebody (maybe Lee Sauder?) told us that it doesn't work like that, you need a naturally high-P ore source to get it in the iron in appreciable levels. That doesn't stop me from thinking about it, though! :lol: I do not know of any experiments along those lines to see if it could work, and of course the preindustrial smelters in this country did not actively try to get phosphoric iron due the various issues that arise with an overabundance of P. You even find references to the pig iron from certain 19th-century furnaces being unsuitable for certain uses due to high P levels. If they'd only known that a century or two later some of us crazons would actually like to play with the stuff to better understand Migration Era and Viking Age pattern welding... ;)

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There was some reference I found a while back to the use of a particular type of tree in India in which the charcoal was being made for the Delhi iron pillar. As most folks who are into this sort of thing know.. that iron pillar is phosphoric iron. Apparently that tree is pretty high in phosphorus and the implication was that this was the reason for high P in the iron... I wish I could find that reference!

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Okay, I suppose my expectation was that it would be a lower temp flux like borax. I was aware that FeO is a flux, but I discounted it because it doesn't melt until a a fair bit higher than what I was assuming was desirable.



Scott, that's very encouraging information!

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There seems to be enough K2O in the rice straw to hold it together at an orange heat and to make it stick to the hot metal ...the other beauty of clean unalloyed steel is..high temperatures are no problem..you can get up to 1.5% C and still be above the melting temperature of that flux and the metal will not fail. FeO/SiO2 flux is very fluid and if there is any carbon under the surface it will draw that out and bubble...this gas pushes the crud out of the weld zone.


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