Before you use Zechstein Minerals flakes to make your mag oil read what they say...http://www.zechsteininside.com/productseng.html "Zechstein Inside" quality. Also look into the pH of the Zechstein products. For my simple test they run on 3.8 pH (+/-). Switch to nigari flakes a high 5.3 (=/-) pH. The skin likes 4 to 6.

Fro their site...
"We do not recommend using flakes as a basis for making high concentrated magnesiumoil. The flakes are produced by an evaporating process. The brine from the source is heated up to approximately 160° C and the water is evaporated till a concentration of 47% of magnesium chloride is reached. This concentration represents the right proportions to get magnesium chloride hexahydrate [ MgCl2.6(H2O) ] which gives a stable flake or pellet after crystallization on a cooling-belt. Due to heating for evaporation a very small amount of MgCl2 will decompose to MgOHCl (magnesium hydroxide chloride), MgCO3 (magnesium carbonate) and some HCl (hydrochloric acid) in the vapor stream, on parts per million level. These products are responsible for the white haze that can be seen after dissolving the flakes. These products are not hazardous but are products of the necessary production step to get flakes. For applications up to 5%, such as bath applications (approx 2-8 kg in a bath) the flakes are perfect.
For topical applications with higher concentrations, we recommend the pure natural magnesiumoil straight from the source without any heat treatment. This magnesium oil is especially selected for this application and is secured by the Zechstein Inside batch protocol.