The glycerin layer from a biodiesel batch can go into soap, but it is not half-made soap: it contains methanol, which makes flammable and toxic vapour when heated, and an unmeasured amount of lye. Remove the methanol first, or do not use the layer at all, then stir it into an ordinary soap batch in small amounts and test the cured bar for free lye.
Part of Biodiesel Production
Soap making from biodiesel glycerin is using the glycerol-and-alkali layer drained from a batch as an ingredient in a soap recipe.
Why The Parent’s Shortcut Does Not Work
The parent says crude glycerin with residual lye is “already halfway to soap”: add water, heat gently, and the soap separates. What the layer actually holds (Glycerin Byproduct) is mostly glycerol, with some soap that formed from free fatty acids, leftover lye in an unknown amount, a large share of the excess methanol, and water. Soap is made by combining fat or oil with lye (Saponification); the layer contains almost no fat, so heating it does not make a bar. The two things it does have, glycerol and alkali, are both useful in a real soap batch if the methanol is out.
What You Need
- Crude glycerin, from a batch that passed the usual settling
- A way to remove methanol safely, or a decision not to use the layer. Methanol boils at 64.7°C. Removing it means warming the layer above that in a closed vessel with a condenser and a receiver, the same set-up as a still (Still Construction, Condenser Coil), outdoors and with every joint sealed. The distillate is nearly pure methanol: label it poison and keep it, or re-use it in the next biodiesel batch
- A normal soap recipe from the corpus (Soap Making, Fat Selection)
- Soap testing means: red cabbage extract or pH strips (Quality Control, pH Testing)
Method
Step 1: Strip the methanol, or stop. If you cannot do this with a sealed, condensed, outdoor apparatus and gear, do not heat the layer: store it sealed and labelled.
Step 2: Make a normal batch of soap to the recipe, with the fat and lye measured for that recipe alone.
Step 3: Add a small amount of the methanol-free glycerin to the batch. How much is a judgement the corpus does not source: start with a few per cent of the weight of fat, and reduce the recipe’s lye a little, because the glycerin brings alkali of unknown strength.
Step 4: Cure and test. After curing, test the bar for free lye. A bar with too much unreacted lye looks normal and burns skin over several washes (Quality Control); if it tests too alkaline, reduce the glycerin or the lye next time, or send the bar for re-batching.
Step 5: Log it. Record the amount and the result so the next batch is better.
Common Mistakes
- Heating the raw layer in a pot indoors. The methanol vapour is toxic and can ignite.
- Treating it as a measured lye solution. It is not; its strength is unknown.
- Using the soap before testing. Free lye burns skin.
- Mixing in the methanol distillate. It is a poison, not soap stock.
Heated glycerin gives off methanol
Methanol is flammable and poisonous through vapour and skin. Never warm the layer in a closed room or near flame. The lye in it burns skin and eyes (Protective Gear, Methanol Hazards).
What’s Next
- Learn next: Other Uses
- Combine with: Hard Bar Soap, for the conventional route to a solid bar