on 17 January 2013
In my usual ore mix, the big pieces are bean size. From that, down to near dust. I think the mix of sizes keeps the flow down healthy.
Back when I used the Spanish red ore, it was very fine, but I'm not sure I knew enough then, to know if it mattered.
I've come to believe it all matters. The size of all the components, to the stack size, to the airflow/volume. But, there are always so many variables with any given smelt, that doing, what has worked well for you in the past, seems to be my best plan these days. Unless I'm experimenting, I stick with what has worked the best, for the ore I'm using that day.
Blooming is an art. There is way to much science going on in the whole process to worry about it. ...>
Mark - I especially agree with your statement "Blooming is an Art".
Beware those that suggest there is a cookbook approach that can be employed. The experienced know there are just way to many variables in play!
Again with the patience of those with more experience reading here...
Lee Sauder has gently reminded me on a number of occasions not to stress 'stack height' too much, because the reality is that it is *hang time* which is important. (Stack height here being the measurement from the top of the tuyere to the top of the furnace.)
As had correctly suggested, for an individual ore body, there is an ideal relationship between particle size and hang time. The reduction chemistry is a gas onto a surface reaction. If you make your ore particles too large (or the ore is too dense and solid), there will be more time required in the working column of the furnace for the desired conversion of iron oxide to metallic iron to take place. The converse of this is that if those particles of metallic iron remain in the carbon rich column for too long, they will absorb excess carbon, and un-desirable *cast iron* will be the product.
The historic purpose of direct bloomery furnaces was to produce an extremely low carbon iron - an easily forgeable metal for general purpose applications. Although many reading here are bladesmiths, you never should forget that truth. As you have seen from the postings of various smelt workers here, it is possible to create a higher carbon metal, suitable for cutting tools. Looking at historic practice however, it is extremely difficult to see if this was ever done intentionally. The evidence appears to point to the use of re-melting hearths being used for carbon/iron alloy production. (Check the other thread : 'Evenstad Experiments')
Back to particle size :
One very important limitation on what people may suggest here : Consider the ore type, quality and source.
We all are generally working with some locally available materials as our ore. Hematite blasting grit (1 - 2 mm of 98% Fe2O3) is going to require a much different furnace arrangement to that needed for working with 'Spanish Red' potter's oxide (powder at 81 % Fe2O3) or ... (insert your own ore type here).
I can tell you from my own experience. My early friendship with Lee and Skip Williams resulted in me often hauling 500 lbs of their (excellent) Limonite rock ore home from Lexington to my base in Central Ontario (roughly 85% FeO). So my 'working' furnace system was designed around the use of a rock based ore, roasted and crushed, with pieces ranging from 1 - 5 mm (rice to pea). Most typically our furnaces had 40 - 45 cm stack height, with air volumes adjusted for an optimal 8 - 10 minutes per 2 kg charcoal **. This produced a 'hang time' of something about 30 minutes *. The end result was typically a nice dense workable iron bloom (when I managed not to screw something else up.) Using the hematite grit in the same furnace set up, the end result was a very crumbly texture, but a medium to high carbon metal. (And yes, small amounts of cast iron was sometimes the result as well.)
Because both Jur and Mark have mentioned this - our standard here, with the more solid ore types (various source limonite rock, industrial taconite) is to aim for 'pea to rice' - and roasted. I have only had a couple of chances to work with primary bog ore (both roasted and not), sizes there more pea to walnut (5 - 25 mm). Although we are using slightly different ways to describe this, I do see we are all working in the same ranges. The only difference I see with what Jur has described is that I normally keep the smaller 'dust' particles included (as normally does Lee, I believe Jesus). I never worry about washing these ores, figuring what sand / rock remains will just give me nice juicy slag, which I can just tap off.
Jur has asked about using the various potter's oxides.
We have used both the 'Spanish Red' and the higher iron concentration (93% Fe3O4) black. The problem with attempting to use these powders 'straight out of the bag' is that the particle size is so fine, the air blast blows almost as much back out of the top of the furnace as you manage to get into the furnace! The solution we developed (our 'DARC Dirt' analogs) is to mix 10% whole wheat flour in with the oxide. Water is added to create a dough, which is then spread on sheets to dry. The resulting 'cookie' is broken up into pieces. The flour acts as a binder, holding the oxide together until it falls into the top of the furnace. This pretty much eliminates the problem of the oxide blowing out of the furnace.
DARC Dirt 2 analog - from black potters oxide (scale inches / cm)
* That is *very* rough - based on estimating the volume of the furnace X ideal consumption rate.
** I'm gathering data for a paper to be delivered this spring. One of the problems is trying to find some 'universal' measurement that can be used that links back to air systems / volumes (where my interest lies here). It does appear that 'consumption time against charcoal' may be the most commonly recorded measurement.