There are three ways to end up with a bar of soap, and they aren't three equivalent hobbies to pick between. The useful way to see them is this: two of the three are the same craft, and the third is a different one wearing the same word.
What's different
| Cold process | Hot process | Melt and pour | |
|---|---|---|---|
| Do you handle lye? | Yes, every batch. | Yes, every batch. | No. The base arrived already saponified. |
| Usable in | 4–6 weeks. | Sources disagree: from a week or two to four weeks or more. The lye is spent once the cook zap-tests clean. | Hours. |
| Control over the formula | Total. Every oil, the superfat, the water. | Total. The same decisions as cold process. | None. The oils were chosen for you. |
| What it looks like | Smooth; fine swirls and layers in the batter. | Rustic. Thick batter fills a mold like mashed potato. | Very clean edges, transparent bases, glitter that shows. |
| Fragrance | Can accelerate, rice or seize. Test everything. | Added after the cook, so nothing to seize, but heat drives off delicate notes. | Behaves. Limited by the base maker’s maximum. |
| Equipment | Scale, stick blender, mold, PPE. | The same, plus a slow cooker. | A knife, a microwave and a mold. |
| Hardest part | Getting the numbers right, and everything the fragrance might do. | Judging the cook, and getting thick batter into a mold neatly. | Sweating in humidity, and additives sinking. |
"Usable in" is the usual wait before a bar is used or given away. All three keep improving afterward for the reasons in the cure guide.
Hot process is cold process, cooked
These two are not separate disciplines with separate recipes. Both dissolve lye in water, both mix it into oils, both saponify. The only difference is that hot process applies outside heat (a slow cooker, usually), so the reaction runs to completion in an hour or two in the pot instead of over a day or more in the mold.
Our classic bar needs 143.8 g of sodium hydroxide per kilo of oils. That is true whether you pour it into a mold or cook it in a slow cooker: same fats, same saponification value, same superfat, same hardness and lather numbers. Every lye calculation on this site applies to hot process unchanged. The setting that usually changes is the water: hot process is commonly made at full water, such as a 25% solution (431 g here, against 292 g at the 33% used for cold process on this page), because the cook drives some off and the batter has to stay workable. What does differ is the working experience: you lose the swirls and gain a batch that has finished saponifying when the lid comes off.
That has one practical consequence worth knowing before you need it. Because hot process adds the fragrance after saponification, there is no reaction left for a difficult fragrance to accelerate. That makes it the answer to a scent that seizes every time in cold process, and the rescue for a batch that has already seized. A slow cooker is a cheap insurance policy for a cold process soapmaker.
Melt and pour is a supply chain, not a method
Melt and pour starts with a base that a factory already made: the saponification happened elsewhere, months ago, to someone else's recipe. You melt it, color and scent it, and pour. That is a real and legitimate craft. It is where the transparent bars and the embedded shapes come from, it does clean layers well, and it is safe enough to do with children.
But the decisions this site exists to help with have already been taken. The clearest way to show that is our own tool list:
| Tool | Cold process | Hot process | Melt and pour |
|---|---|---|---|
| Recipe Builder | ✓ | ✓ | — |
| Lye Calculator | ✓ | ✓ | — |
| Mold Volume | ✓ | ✓ | ✓ |
| Batch Resizer | ✓ | ✓ | ~ |
| Superfat Adjuster | ✓ | ✓ | — |
| Lye Concentration Converter | ✓ | ✓ | — |
| Masterbatch Lye | ✓ | ✓ | — |
| Liquid Soap Calculator | ~ | ✓ | — |
| Dual-Lye Calculator | ✓ | ✓ | — |
| Milk Soap Calculator | ✓ | ✓ | — |
| Salt Bar & Brine Soap | ✓ | ✓ | — |
| Melt and Pour Calculator | — | — | ✓ |
| Fragrance Load (Soap) | ✓ | ✓ | ✓ |
| Essential Oil Blend | ✓ | ✓ | ✓ |
| Additive Usage Rates | ✓ | ✓ | ~ |
| Citric Acid & Vinegar Lye | ✓ | ✓ | — |
| Oil Substitution | ✓ | ✓ | — |
| Rebatch Calculator | ✓ | ✓ | — |
| Soap Cost & Pricing | ✓ | ✓ | ✓ |
| Bar Count & Cut | ✓ | ✓ | ✓ |
Rendered from this site's tool registry, so it stays true as tools are added. ✓ applies · ~ partly · — nothing to calculate.
8 of 20 tools have anything to say about melt and pour, and the ones that do are about molds, money and scent, not about soap (the melt and pour calculator puts those three together). Superfat, water discount, oil selection, lye: all fixed before the base reached you. If what draws you to soapmaking is deciding what goes in the bar, melt and pour will feel like decorating someone else's cake. If what draws you is the decorating, it is the only method that does it properly.
What a bar costs
Melt and pour is cheaper to start (no scale worth the name, no goggles, no stick blender), and that is how it is usually sold. Per bar, on the example prices below, it is the expensive one:
| Per 100 g of finished soap | Cold or hot process | Melt and pour |
|---|---|---|
| Oils, or the base | $0.54 | $1.20 |
| Lye | $0.12 | — |
| Fragrance at 3% | $0.19 | $0.24 |
| Ingredients | $0.85 | $1.44 |
Ingredient costs for the classic bar at 1,000 g of oils, 5% superfat and a 33% lye solution, using the same illustrative bulk prices as the cost calculator's worked example: olive $8.00, coconut $6.00, palm $5.00, castor $12.00, lye $10.00 and fragrance $80.00 per kg, with melt and pour base at $12.00. Cold process is costed against its cured weight (1,247 g from a 1,466 g batch), because that is what you sell; melt and pour loses nothing. Prices vary enormously, so put your own into the tool.
$0.85 against $1.44 per 100 g of finished soap on these prices. The gap is not a markup on convenience so much as what you're buying: with cold process you buy the fats and the lye, and the water (20% of the batter by weight) is free. About 75% of that water evaporates during the cure, so it adds little to the weight you end up with; the saving is in buying fats rather than soap. With melt and pour you buy the finished soap, glycerin, solvents and all, by the kilo, from someone who has already paid for the fats and the labor. Cheap to begin and expensive to scale, which matters most to people thinking about selling.
And rebatch, the fourth one
Grate a finished cold process bar, melt it with a splash of liquid, and remold it. A plain rebatch has no lye handling, no calculation, and the same "you can't change the recipe" limit as melt and pour, because the soap is already soap. It exists for two reasons: to rescue a batch that went wrong (a lye-heavy loaf that needs oil stirred through it, or a wet one that needs water driven off), and to make something rustic on purpose. A rescue does need numbers: the oil a lye-heavy loaf needs, or the lye a greasy one is short, has to be worked out, which is what the rebatch calculator does. It is not a starting method, but it is the reason a failed cold process batch is rarely a wasted one.
So which should you do?
- Start with melt and pour if you want soap this weekend, you're making it with children, or the appeal is the design rather than the formulation.
- Start with cold process if you want to choose what's in the bar, you intend to sell eventually, or the chemistry is part of the draw. The first-batch walkthrough is written for exactly this.
- Add hot process when a fragrance defeats you, a batch seizes, or you want bars you can use sooner. It is not a separate skill to learn. It is the same oils and lye with a slow cooker.
- You do not have to choose once. Plenty of makers sell cold process bars and melt and pour novelties from the same table, for the same reason a bakery sells bread and cake.