For availability, color selecton and cost-effectiveness, crayons are king in soap making.  However, they also present a few challenges.  The first of these would be how to measure?  Add a dollar-store, hand-held grater to the list of thngs you'll need.  You can start crayon coloring by adding around 1/8 teaspoon of crayon gratings to one once of soap base.  This is roughly equivalent to 10 passes of the crayon across the grater.  You can add the gratings prior to melting the soap base, or immediately thereafter, as you are essentially melting the crayon as well.  Grating the crayon onto a napkin, paper towel, paper plate or piece of paper allows for effective funneling of the gratings directly into your measuring cup.
For blues an most browns, crayons were a joy to discover!  The liquid blue is a touch too green to me.  Crayons allow for those illusive country blues as well as more truly sky blues.  For browns, my liquid color mixng skills are virtually non-existent.  Still, with crayons, I was a bit dismayed to discover tht browns with dominant orange or red undertones tend to present as orange or red in opaque soap base.  Crayons also provide for other neutrals such as gray, cream, almond and beige.
Though generally happy with  blues and browns, the crayon reds made me weep.  The picture to the right is of crayon color cubes.  The color of the crayon (as named by the manufacturer) is given on the color cube.  Of particular note are the MAROON and FUZZY WUZZY cubes.  Most of you are familiar wth maroon an can clearly see how it bled to a deep orange.  FUZZY WUZZY, whom you may recall, ". . . was a bear", was a lovely brown with some red undertones.  When melted into the shea butter glycerin (as all of these were), it bled to the red.

Though this demonstrates the importance of testing crayons for color change, you can as see how they dramatically expand the pallet of colors available for soap making.  I passed a crayon over the grater 10 times (over a paper towel) and funneled the gratings into the measuring cup before adding approximately on ounce of the soap base.  You can see how even this scant amount of crayon yielded a good depth of color.

Crayon reds, even darker burgundies and maroons, bleed to orange in opaque soap base.  As you might expect, black becomes gray in white soap base and crayon opacty brings cloudiness to any clear soap
There can be a vast difference in the pigment quality of crayons (depending on manufacturer and sometimes color), so do a test.  On a white sheet of paper, start coloring!  If you get more milky wax than pigment, such will be the case with your soap.

Enhance the metallic qualities of silver-, brass-, copper-, and gold-colored crayons by adding a little pearl powder or a matching or complimentary color of soap glitter to the melted and crayon-colored soap base.

Crayon can stain!  Treat those little crayon gratings carefully.  I was surprised to find that a smudged crayon grating resulted in a hard-to-remove stain.  If a grating "escapes" from the paper towel on which it was grated, lightly moisten your finger tip and retrieve it wth a light touch.  This is a good reason to make sure your work surface is protected.

Over-adding crayon is, in effect, over-adding wax to your soap.  Not only is this a potential problem with staining, it can also diminish the lathering qualities of the glycerin or result in a rather crumbly soap.  On the plus side, most crayons are labeled with color name.  Of course, they are fairly discernible visually, but the label always helps.  You can't re-create a soap if you don't know the color used.

Once you've used crayons in soap making for a while, you may find grating unnecessary and may prefer to cut or snap off a length, depending on the volume of soap you are making.

I hope that this information will be sufficient to inspire you to at least experiment a little with the use of crayons to color soap.  You have the tools needed to avoid the major "pitfalls", and use of color cubes will not only help in identifying potential color bleed disasters, but you'll be delighted at how much, how often and in how many ways those color cubes can be used.

SPECIAL NOTE:  You should be aware that Crayola, for one, does not condone the use of crayon colorants in the making of any cosmetic or soap product.  They stipulate that crayons are non-toxic, but want to be clear that crayons have not been clinically tested for "use on skin".  

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