Debunking a wives tale about the value of steam
Below is a proofed demi-baguette that was marked with lines spaced 1.25" apart. It is about to be baked without any steam as a baseline for testing the hypothesis that steam facilitates the stretching of dough. Since this loaf is not scored, we should expect it to blow out along the side. But still, if the surface stretches in response to internal pressure generated by the expanding CO2, then we will be able to observe and measure how much stretch there is.
Here is the resulting baked demi-baguette. The spacing between the lines is still very close to 1.25" indicating that there is little or no stretching when baked in a dry oven (this was baked in a combi oven set to hold the box humidity below 20% which effectively removes even the steam that escapes from the bread itself).
The photo below is another demi-baguette from the same batch that was baked with steam. It too was marked with lines spaced 1.25" apart before it was baked. This loaf had a defect on the top that allowed it to open slightly (actually the side-to-side dimension of the slit it almost exactly 0.25"), and the post-bake line spacing is very close to 1.375" except where the defect increases it to 1.625". So there is some small amount of surface stretching that seems to be facilitated by steam in the oven.
This loaf was baked in the same combi oven but with the steam generator and humidity controls set to maintain 100% humidity in the oven for the first 7 minutes of the bake (when it was just beginning to brown).
The photo below is another demi-baguette from the same batch that was slashed and baked with steam, illustrating the surface expansion that occurs when a well proofed loaf is slashed to allow the oven spring to open the loaf where you want it to split.
So the data indicates that the difference between having no steam and maximum steam is the difference between no surface area increase, and perhaps ~20% area increase even when there is no steam in the oven. It is a measurable but not significant effect. However, you can see the difference in color between the steamed and un-steamed loaves, with the steamed loaves having a more yellowish tone and a shiny surface (as opposed to a dull brown surface for the loaf baked without steam.
It is worth noting that this experiment has a sample size of 1 which does not imbue it with great weight in a statistical sense. But it does set expectations and will guide further experimentation. This particular batch of dough was mixed at 70% hydration, which is a bit higher than the 67% at which I would normally make baguettes. The objective was to build a fairly stretchy dough that I thought might be more amenable to surface stretch than a lower hydration mix. The next step up would have been 75%, but at that level it is ciabatta and I was not sure that I could put marks on the surface without deflating it.