By volume. (Though my mind also goes to the object of this exercise and I do my pizza dough by weight.) Unlike flour, these are all fairly stable materials that will settle out the same way no matter how many times you scoop up a measure. That’s versus flour, which compresses or expands depending on how much air you mix into it.
How much material do you need?
Your oven’s installation instructions will tell you how much premixed dry high-temp mortar you need. If you are planning to use the 4-material blend, and are thinking “How much of each of these materials do I need?” (like I was), here’s a cheat sheet to figure it out:
Take the total quantity of dry premix specified for your build and divide it by 6 (which is the total number of parts you need in the mix ratio).
Then, multiply the result by the number of parts needed for each ingredient. (Which requires only one multiple, for the sand, because every other ingredient is just one part!)
For my build, the Mattone Barile Grande, the instructions specfiy about 400 pounds of dry pre-mix. So:
400 ÷ 6 = 66⅓ (or 66.667) pounds
So the approximate quantities of dry ingredients I need are:
Silica Sand: 3 [parts] X 66.67 = 200 pounds
Fireclay, Lime, and Portland Cement: 1 [part] X 66.67 = 66.67 pounds of each ingredient
Of course you’ll need to round up to the closest bag size they sell, but this should also account for any differences in volume among the different materials.