In a small bowl, dissolve yeast with 1/3 cup of the warm milk. Set aside until foamy (5-10 minutes).
In a large bowl, add flour. Make a well in the center of the flour and pour in both the yeast mixture and the remaining warm milk. Beat for at least 5 minutes. Cover the bowl and let the batter rise in a warm place for 1 hour.
In a small bowl, dissolve baking soda in the warm water. Add to the flour mixture, beating it in thoroughly. Cover bowl and let rise in a warm place for another hour.
Grease a griddle and crumpet rings with melted shortening. Place the crumpet rings on the griddle and heat the griddle to 275ºF or until a drop of water sprinkled on the griddle sizzles.
Pour enough batter into the crumpet rings to fill the rings half full. Cook the crumpets for about 5 minutes. Turn over each crumpet when the surface loses its liquid appearance. Cook for an additional 1-2 minutes.
Remove the crumpets from the griddle, ease off the rings. Continue filling rings and cooking until all batter is used. Serve warm with plenty of butter.
NOTE: To achieve the proper degree of honeycombing, a crumpet batter must be thin enough to let some bubbles rise to the surface during cooking. If none appear, add a little more liquid to the batter.