[Sca-cooks] Use of Soda in period

[Stanza693 at wmconnect.com]

Greetings to the list.

Please forgive me if this happens to show up twice, but I think it was 
bounced back to me the first time I tried it a couple of days ago.

I am starting to work on my next A&S entry, a redaction from the Manual de 
Mugeres, that is a type of fry-bread, sort of.  (The bun~uelos recipe)  The 
problem with cooking bread in period is that *everyone* is supposed to know how it 
is done so no one wrote a recipe.  

In researching leavening agents, I ran across a reference to using soda, 
cream of tartar, flour and water, to make leavening -- and my assumption, is a 
starter.  What would be a modern equivalent to soda?  Or better yet, what would 
be the period form of soda?  I know baking soda is thoroughly modern and quite 
frowned upon in the Outlands, so, I'm perplexed.

I probably won't go with that idea.  I will probably use live yeast, but the 
concept intrigues me.  The reference mentions using plant ash to render soda.  
Sounds fairly caustic to me.

Thanks for any advice you all can offer a new cook!!
Constanza Marina de Huelva