This is where we veer a bit from the beaten path. Few of the following could be considered "basic" marbling techniques. Nonetheless, they are basic from the standpoint that many of the more advanced techniques (available in the online store) and designs require having at least a passing knowledge of these processes. More importantly, they provide you with multiple alternatives for achieving beautiful, dramatic, marbled soaps.
All of those color cubes that have been or will be made by pouring out leftover soaps or made to check for color saturation will serve many purposes. This is one of them. Unmold the color cubes as you would ice cubes. Should the color cubes prove more resistant to unmolding, try sliding a craft stick down the side, between the color cube and the ice tray. This will often pop the cube from the tray.
You can approximate volume by the depth of the cube since one cube compartment holds roughly one ounce (in most standard ice cube trays). Choose which colors you want to see marbled together, making sure you have sufficient volume to fill the mold. If you are using unscented color cubes, pick one color to melt first so that the fragrance oil may be added and blended. If the color cubes are made from scented leftovers, then place all the cubers you're using into the microwave-safe measuring cup at the same time. Microwave for 10-15 seconds to melt. Microwave for another 10-15 seconds if not completely melted. Avoid the rising, boiling action in the microwave is possible. This tends to over-blend the different colors. Watch the soap as it melts. Watch for tell-tale bubbles forming at the top. This will normally alert you to when boiling is about to begin. Remove the cup from the microwave and allow to sit for a few minutes to cool. DO NOT STIR! Once you see a film forming, it's time to pour the mold. As there may be an unwanted film on top, use a craft stick to hold it in the measuring cup while you pour the melted soap. Allow a film to form on your poured soap and freeze-set as usual. Cutting the color cubes into smaller pieces and altering placement in the cup will better distribute various colors throughout the soap . . . but you also run a greater risk of over-blending during the melting process.
Keep in mind, the inability to stir-cool the soap dramatically increases the possibility you'll warp a mold. Until you get a feel for this technique, I strongly recommend limiting its use to cookie cutter pours (which will be covered soon).
In-Mold Marbling (Clear Soap Bases Only)
With in-mold marbling, you melt and pour your scented and cooled soap base into the mold and then, using liquid colorants, put a drop or two on top of the poured soap, spacing them away from one another. Then, using a toothpick and a curly, swirly (or zip-zag) stirring motion, you drag the colorant through the soap. I find that when I use this technique, I tend to like the bottom of the soap more than the top as the marbled veins sometimes don't reach the bottom of the mold (top of the soap). However, using this method with a clear soap base allows visibility of the marbling through the soap, greatly enhancing the overall effect.
Ink Stain In-Mold Marbling (Clear Soap Bases Only)
Though the name would imply otherwise, you won't be using "ink" to marble your soap and you won't be using opaque or white soap base. This technique is lost unless you use a clear glycerin base. That, of course, means you won't be freeze-setting the soap because clouding this effect would be a crying shame.
Melt and scent your clear base and pour into your mold. With clear, liquid colorants at the ready, use the same one-drop method previously used for in-mold marbling. However, don't stir or drag a toothpick, or anything else, through the soap. Just allow the drop of colorant to bloom naturally across and through the soap. This method can be really dramatic in its results, but it is also tricky in its timing. If you are using a soap mold, you have to stir the melted soap (in the measuring cup) long enough to cool so the mold won't warp. however, if the soap is too cool when you pour up the mold (if a film has formed on the soap in the mold), the liquid colorant has a tendency to just sit in a tiny little pool on top of the soap. Should this happen, employ the toothpick and the curly, swirly stirring to drag the colorant down into the body of the soap. This won't result in the same effect, but will (as shown above) marble the soap base.
Also, with any clear soap marbling, you'll notice that when you mist the top of the poured soap with alcohol, it moves the liquid colorant around. I haven't been able to determine if the colorant is moving toward or away from the thinly pooled alcohol, but you can easily see the movement when you spray. This can be used as a final "marbling" tool, to some degree, but may also result in a little frustration when the colorant moves in or out of an area where you were well-satisfied with the effect.
I do not normally use this method because I am not particularly ambidextrous, but I want you to have the opportunity to experiment with it. As the name I gave it implies, you need both hands (or a helping hand) and at least two microwave-safe measuring cups. Simultaneously melt, color and scent soap base in two cups. For marbling, you obviously need to use a different colorant in each cup. Once the two colors of soap are melted and stir-cooled, take a cup in each hand and simultaneously pour the soaps into the same mold, usually from opposite sides of the mold. I have seen pictures of soaps that use one clear and one opaque soap for an interesting effect. However, unless you move the cups back and forth across the mold, the colors tend to stay segregated on either side, with the only (limited) blending occurring where the two colors meet.
Set Soap Marbling
This is another technique that I discovered quite by accident. It also makes excellent use of the color cubes and leftovers from other soap projects. Using a craft stick, scrape across the surface of the cube without unmolding it from the ice cube tray, peeling a strip of the set soap from the cube. Do this for as much soap in as many colors as you like. Using your fingers, smoosh and mush the soap back together, blending as much or as little as you want. From there, you can use the marbled portion of the soap as an embed or to "pack" the raised design portion of a decorative soap mold. This is discussed in much greater detail in the downloads which are available in the online store. This overview simply illustrates the many options you have when it comes to marbling soap.
Regardless of the soap base, colorant or marbling technique you most enjoy using, I hope this gives you a good launching point for experimentation. The remnant marbling and set soap marbling techniques are discoveries I made and these techniques are just two of the processes that are uniquely my own . . . discovered through trial and error. It's not difficult for me to imagine that there are still other marbling possibilities out there . . . ones I hope you discover and develop and share.