Not safe to use additives for melt and pour:
Almost all biologicals, as they will rehydrate in melt and pour and thus spoil. This is a health hazard, whether you can see it or not. This is different when making cold or hot process soap, as then the fresh or dried ingredients go through the saponification process.
So unfortunately you cannot use things like rose petals, lavender buds, any kind of tea (including herbal tea and matcha), any kind of fruit/vegetable/flower/herb (dried or not, powderderd or not), basically any kind of fresh and dried ingredients like that, aside from a very select few, see below.
Also not safe: ingredients with a high water content, like rose water, juices, purees or milks.
Safe to use additives for melt and pour:
- Liquid dye (provided it is soap safe and skin safe).
- Mica (provided it is soap safe and skin safe).
- Fragrance oils (same). Note: see maximum safe useage rate below.
- Essential oils (except cinnamon). Note: see maximum safe useage rate below.
- Vanilla stabiliser (needed when adding a FO or EO containing vanilla/vanillin). Note: see maximum safe useage rate below.
- Color block system (as sold by soap shops).
- Activated charcoal (not the charcoal sold for teeth though because of other ingredients).
- Oxides sold for soapmaking, like titanium dioxide.
- Ground or whole old fashioned or colloidal oats.
- Used very dry coffee grounds. Note: these *might* spoil at a later date anyway. YMMV.
- Turmeric powder.
- Ground walnut shell.
- Dried calendula/marigold petals.
- Dried cornflower petals. Note: these *might* turn brown at a later date anyway. YMMV.
- Skin safe glitter.
- Dried loofah.
- Various clays (as sold by soap shops).
- Bee pollen.
- Honey. Note: see maximum safe useage rate below.
- Shea butter. Note: see maximum safe useage rate below.
- Coconut oil. Note: see maximum safe useage rate below.
- Glycerin. Note: see maximum safe useage rate below.
- Silicon, plastic or resin toys/flowers.
- Water soluble paper.
- Temporary tattoos. Note: some of these will bleed immediately or at a later date. YMMV.
- For the soap base itself the safe maximum of liquid/butters/oils is 1 flat teaspoon per pound/500g of base (excluding alcohol as that evaporates).
- For FO and EO you need the safe usage percentage from the manufacturer. If that is more than 1 teaspoon per pound of base, use the 1 teaspoon maximum.
- For additives like honey, shea butter or coconut oil it's likely a better option to buy a base with the additive you want already in it.
- For biologicals like aloe vera, cucumber, carrot etc. your only safe option is buy a base that has that is the additive you want already in it.
- Not all additives might be usable/deemed safe if you sell your soaps, that mostly depends on where you live, so do your research.
- Too much "stuff" in a sink like glitter, oatmeal, cornflower petals can be annoying and can also clog the drain.
- Added glycerin is usually not needed at all as melt and pour soap IS glycerin soap.