Generally speaking, include less of what is making it hard/thick, and more liquid/soft ingredients.
Hardening/thickening ingredients are typically waxes (beeswax, candelilla wax, cera bellina, etc.) or fatty thickeners like stearic acid or cetyl alcohol. In an anhydrous (no water) product these ingredients typically provide the bulk of the thickening power. It is also possible that the thickening comes entirely from the blend of butters—if a product is approximately 40% or more solid or brittle butter (cocoa butter, mango butter, shea butter, etc.), that can be where the firmness of the product comes from without any additional thickeners.
The viscosity of an emulsion is influenced by the size of its inner phase. The lotions we usually create at home tend to be oil-in-water emulsions, so in that case the larger the oil (internal) phase, the thicker the end product. An oil-in-water lotion with a 25–30% oil phase will be significantly thicker than a lotion with a 15% oil phase, even if the 15% oil phase contains other thickeners like cetyl alcohol or gums. While you can reduce the thickness of a lotion by using liquid oils instead of solid butters and/or thickeners, the best way is to reduce the size of the oil phase, which will require re-calculating the recipe so the amount of emulsifier and water remain in balance. I cover how to do this here. Please keep in mind that the stability of emulsions is often influenced by the viscosity; if you drastically thin out an emulsion you may encounter stability issues.
Watery products that aren’t emulsions often get their viscosity from gelling agents and gums like xanthan or hydroxyethylcellulose. To make such products less viscous simply use less of the gum and more water.
Remember: never just decrease or delete an ingredient. You must keep the formula in balance by ensuring it always adds up to 100%. If you are aiming to make a thinner/softer product this will typically mean replacing the thickener you’ve removed with something liquid, like water or liquid oil.
Posted in: Troubleshooting & Adjusting