Iron ore processing

[Barbarian]

As well as beekeeping, I am a member of a local amateur archaeological group.

One of our ongoing projects is working on a medieval iron bloomery. We have come across an extensive whitish/grey clayey layer. Does anyone know what this might be ? The local ironores are haematite and siderite.
I am wondering if the deposit is produced when the ore is roasted ?

Thank you,

 

[alldigging]

Does GMAU not know?

 

[madasafish]

https://en.wikipedia.org/wiki/Bloomery may help

 

[Barbarian]

Thanks for the responses.

The Greater Manchester Archaeological Unit was closed down several years ago.

The wikipedia entry is interesting but not totally accurate about the stages in the process.

I will explain the method later.

 

[REDWOOD]

Silicon dioxide, I believe it was used to purify the smelting process. Examples were found in Carthaginian iron works and was originally believed that by adding silicone to purify the metal was a modem discovery

 

[Barbarian]

Thank you Redwood. I will read up about Silicon Dioxide.

From what I understand about the local bloomery site, the ore was broken down into small bits. The small bits allowed the heat of the next stage, roasting, to penetrate the bits.
The roasting changed the ore into iron oxide .... red bits.
The furnace was lit and charged with charcoal. Air was pumped into the furnace with a large manual bellows. The charcoal burned and the temperature rose. More charcoal was added at the open top of the furnace. When the temperature was right the next stage started.
Gradually, a mix of iron oxide and similar sized bits of charcoal was added at the top of the furnace. This mixture and the temperature of the furnace produced a layer that was reducing. The iron oxide was changed to molten iron and gaseous carbon monoxide.
After several (?) hours the furnace was tapped. A hole was made in the furnace base and a slag mixture flowed out into a tapping pit. The semi-molten iron (bloom) remained in the furnace base.