The bloom value or bloom strength of gelatin is a measure of its hardness. For photographic and holographic purposes, a bloom value in the range of 200-300 is common. Anything less than 200 tends to be too soft, and over 300, too hard. Many holographers in the US start with Knox brand gelatin (available at most grocery store) and get good results.
The following table summarizes the bloom value for some readily available gelatins.
|Great Lakes (bovine)||225|
|Great Lakes (porcine)||225|
|Knox||225 - 235|
Consumer-grade gelatin can vary from lot to lot. The 1-oz. box of Knox gelatin is handy for the random experiment, and it is readily available, but it may not have the consistency from box to box needed for controlled results. Moreover, small quantity purchases get expensive. The 1-oz. box may case around $3.00 US, while a full pound of either Knox or Great Lakes gelatin is less than $20.00 US. Any good baking supply shop should carry the bulk sizes at reasonable prices.