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Thilo is correct on why there is a variation on color. Yes, composition can have an effect, but it is more closely related to oxidation and ion state. I have often seen this when doing experimental smelting and see color changes based on slag temp and alterations in oxidation state. I often wonder how much effect the weather has on the color as well during the tapping process.
Aaron
On Wed, Jul 24, 2013 at 3:06 AM, Rehren, Thilo <[log in to unmask]> wrote:
Dear All,
There is a lot more physics behind the blast furnace slag colours than bulk chemistry. Subtle changes in oxidation state and concentration of the individual chromophor ions (mostly Fe2+ I assume), their surrounding field arrangement, viscosity etc. all play a big role. Particle size (nano clusters of iron metal as well as microscopic inclusions of droplets or crystals) will influence colours, too. These parameters can change within a few mm, less so from within the furnace where conditions should (should...!) be consistent, but during tapping or other removal from the shaft, and incomplete homogenisation as a result of the high viscosity of the melt. Hence your schlieren and different colours in the same piece. Differences in colour therefore are in my view likely to be more a reflection of the very final stage of slag tapping than of the main process itself.During my work on early Islamic crucible steel slag (mostly blue to turquise green) I noticed that the turquise green (ie non-transparent, but still glassy) slag turned purple when it devitrified (in radial arrangements around small imperfections in the melt, such as metal droplets or sand grains). And that was for slag that cooled within the crucible, ie with stable redox conditions.
Bulk chemistry will of course also affect colours, but in order to make assumptions about added fluxes, change in ore etc. I would strongly recommend to get proper chemical analyses done.
As for the bloomery slag, that should indeed be mostly crystalline, but can still appear dense and 'glassy' to the naked eye, in particular if it is cooled quickly and / or washed down a river and water-polished. Again, eye assessment is a starting point, but not sufficient for any reliable statement about chemical composition.
Most importantly: aren't they just beautiful?
Thilo
From: Arch-Metals Group [[log in to unmask]] on behalf of Lyle E. Browning [[log in to unmask]]
Sent: 24 July 2013 05:17
To: [log in to unmask]
Subject: Re: Slag Color Change
Is the bloomer slag glassy? I am a bit confused as I had not thought bloomery slags were glassy (as in solid glass) but were rather more like skulls/mossers from forges with mixtures of charcoal, ferric oxides and silicas.
Tom Brady collected slag samples from all of the Virginia charcoal blast furnaces in his career, more than half of which have unfortunately been discarded. All of the ones I have seen were glass. The earliest furnace in the New World (www.fallingcreekironworks.org) has not been proven to have been in blast before termination by the Powhatans, but literally all of the rest starting in 1715 and working to the 1880's had the solid glass slag. The colors are generally green and blueish green with a few of the brighter blue. There are some that have black to purplish black. I have not observed any white slag, even on the coke fired later examples that I have recorded. I have seen once piece of slag from Connecticut that was ruby red.
My original question seems to have gone in the direction of differences in flux and to differences in operation. So are both right or is there some analytical work needed to see what's what? I downloaded Forsyth's The Blast Furnace and the Manufacture of pig iron and on page 166 he writes about the colors to a point, but he is also referring to coke as the fuel rather than charcoal.
My late colleague James Brothers and I went over this extensively (well, his reading was far more so than mine) and we reached the conclusion that the entire working history of the furnace was contained in the slag and with analysis could be ascertained. Is that a widely held view?
Lyle Browning
On Jul 23, 2013, at 4:26 PM, N PHILLIPS <[log in to unmask]> wrote:
Hi
I am currently working on the Angidy Furnace in Tintern, S. Wales. UK.
The Furnace has a range from 1566-1846 with a lot of wireworking experimentation carried out there. I am getting mainly translucent greens and blues with very occaisional light grey. I see from your replies that you attribute the difference in colours to changes of flux or ore. I have some pieces which have both colours, is that a problem to your theory?
My main question however is whether the glassy green and blue slag is attributeed only to charcoal fired furnaces. Earlier bloomeries in the area appear to produce opaque black (Trelech Monmouthshire, S. Wales, Medieval) whilst the later furnaces depositing along the River Usk, Monmouthshire, S Wales UK tend to have opaque grey blues.
Neil
07734962919
From: Patrick Martin <[log in to unmask]>
To: [log in to unmask]
Sent: Monday, 22 July 2013, 20:03
Subject: Re: Slag Color Change
Jackson furnaces at Fayette in Upper Michigan burned charcoal from 1867-1891 in one, then two, later enlarged and improved furnaces, hot blast for much if not all of that time. There is no evidence I am aware of that the source of ore, fuel, or flux changed significantly through that period, yet the ABUNDANT slag around the site is incredibly variable in color, including green, blue, purple, even reddish, with grey predominant. I suspect much of this variation is due to changes or variability in operational procedures, rather than material inputs.
On Mon, Jul 22, 2013 at 1:41 PM, Killick, David J - (killick) <[log in to unmask]> wrote:
Hi Lyle
I think that the change in color probably corresponds to a change in slag
composition, but this could be attributable to a change of flux (or the
proportions of flux to ore) as well as to a change of ore. In my
experience slags that are light blue or turquoise are fully glassy slags
that are mostly composed of oxides of calcium, magnesium, silicon and
aluminium - the calcium and magnesium deriving from heavy additions of
limestone or dolomite flux. I haven't had much experience with the green
or purple slags that you mention, but the green color might suggest a
higher proportion of iron oxides than in the blue slags.
Dave
On 7/22/13 10:06 AM, "Lyle E. Browning" <[log in to unmask]> wrote:
>Mossy Creek Iron Works was a charcoal fired, cold blast furnace in
>Augusta County, VA, USA that nominally operated from 1774 to 1841 when it
>burned. The typical glassy slag color is dark green, or an almost purple
>or some tint of dull blue. After the associated forge and foundry ceased
>operation in 1880, the property was sold to a grist milling company who
>erected a crib dam in 1903 that had a fair amount of robins egg blue to
>turquoise blue slag. There are also reports that other areas nearby have
>the same used as fill around culverts and so forth. The presumption is
>that the slag was from the ironworks and not imported from elsewhere as
>crib dam fill.
>
>Is there anything that would cause a color shift in the smelting process
>other than a change of ore source?
>
>Thanks in advance,
>
>Lyle Browning
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