1/2 C water
1 C pastry flour
1/2 Tbs molasses
1/2 C water
1/4 tsp pearl ash
1 C pastry flour
1/2 Tbs molasses
1/2 water
1/4 tsp baking soda
1 C pastry flour
1/2 Tbs molasses
1/4 C water
1/4 C barm
1 C pastry flour
1/2 Tbs molasses
1/2 C ale
(carbonated)
1 C pastry flour
1/2 Tbs molasses
1/2 C ale
(non-carbonated)
1 C pastry flour
1/2 Tbs molasses
1/2 C ale (carbonated)
1/4 tsp pearl ash
1 C pastry flour
1/2 Tbs molasses
1/2 C ale
(non-carbonated)
1/4 tsp pearl ash
1 C pastry flour
1/2 Tbs molasses
1/2 C ale (carbonated)
1/4 tsp baking soda
1 C pastry flour
1/2 Tbs molasses
   I have seen many modern "colonial recipes" call for the use of baking soda and/or beer to be used in leavening baked goods.  Baking soda (sodium bicarbonate) was not a known chemical in the colonial era pantry.  Baking soda is a type of alkaline salt that has some basic chemical properties that are similar to the period correct chemical rising agent pearl ash (potassium carbonate), but as you can see in this demonstration they are not equivalent.
     Both pearl ash and baking soda need an acidic ingredient to react with in order to give off carbon dioxide gas which will make the baked good rise.  For all the experiments I used the same quantity of molasses as it is a period ingredient for baked goods.
     Beer is another modern addition to "colonial recipes" that is misunderstood.  While many recipes for cakes and especially breads call for the use of ale-yeast or barm, none call for beer.  Barm is the live yeast cells that are present at the top of fermenting beer.  Once the beer is done fermenting some yeast cells are still present but in a very low quantity.  I made of a batch of barm using dry malt extract, warm water and beer yeast.
         Modern beer is very carbonated and so when added with flour alone you can see that the gas bubbles will raise dough without any effort or additional leavener.  Beer in the 18th century was bottled and casked which would hold in some of the carbonation but not to the extent of modern beer.  For the non-carbonated beer I left the bottle open for 3 hours.  I used Red Speckled Hen from England, a bottle conditioned ale that has some of the yeast in each bottle.  
       Yeast is a simple organism that consumes sugars and as a by-product gives off carbon dioxide and alcohol.  In baked goods the carbon dioxide is trapped in the flour and makes the bread rise and the alcohol burns away leaving that wondrous smell of fresh bread.  In beer the yeast eats the sugars, leaving the alcohol in solution and gives off carbon dioxide which are the fizzy bubbles of a freshly opened beer.
     Another non-period ingredient that many utilize in colonial baking is self-rising flour.      This sort of flour is general purpose flour which already includes baking powder and salt.  Baking powder was not invented nor commonly used in the United States until the mid-1800's.  This chemical leavener is even stronger and has more rising action then baking soda so I did not bother doing experiments with it.  If I did it would be similar if not more exaggerated then the baking soda examples.   For this experiment I used pastry flour that has a moderate gluten level and is milled from soft wheat much like the varieties of wheat available in the 18th century.
     As a constant I baked an example of just water and flour with molasses to give a starting point to compare the leaveners.  The pearl ash was always thoroughly dissolved in the liquid before adding molasses then flour.  The "recipe" for this experiment produced three muffins, each scooped into the muffin tin using a premeasured scoop designed for this size muffin.  I choose the median example from each set to compare.

 
     

An Experiment with
Period & Non-Period Leaveners