“Mechanic soap” usually refers to extra scrubby bars. The exfoliants help remove grease and oil from working hands. For these bars, I added 5 tablespoons of pumice and 8 tablespoons of walnut shells for plenty of exfoliation. Both pumice and walnut shells are effective yet fine grained, so they are not overly scratchy. These bars are be suitable for the body, but I wouldn’t recommend them for gentle facial skin. They are great for rough spots like elbows, knees and heels.
I kept these bars no-fuss with no colorants or fancy design. Sometimes, simple is best. The walnut shells give a natural warm hue. When the orange 10X essential oil is added, it turns the batter a rich orange, but I found it faded as the soap hardened. The pumice does not affect the color greatly, but it does cause the soap batter to accelerate. Prior to adding the pumice at trace, I recommend dispersing it in oil to help get rid of clumps.
I used my melted and mixed soaping oils to disperse the pumice, rather than adding extra oil to the recipe. This is a personal preference. Normally, I use extra lightweight oil to disperse my colorants and consider the dispersion oils a little extra superfat. For this recipe I did not want to add 5 whole tablespoons of extra oil because I wanted these bars to be nice and cleansing (and remember, any extra oils = weigh down lather and make rinse-off not as easy and fast). This recipe is already formulated with a 5% superfat, and contains a 12% water discount to help these bars cure faster. Read more about water discounting soap here.