Platinum vs tin-cure silicone: which is body-safe?
This guide was drafted with AI assistance.
Platinum-cure silicone is the one for anything that touches skin: it cures with no residue, barely shrinks, is non-porous and stays stable for years. Tin-cure silicone is cheaper and more forgiving, and great for molds, but it leaves catalyst residue and degrades, so it is not for finished pieces used on the body. The datasheet tells you which you have. If it does not say, assume it is not suitable.
This is the question that comes up first whenever someone wants to cast their own soft silicone pieces, whether that is a grip, a prosthetic, a squishy toy or an adult toy, and it is the one decision you cannot fix later. Get the material right and the rest is craft. The full making process is in how to make body-safe silicone toys; this page is only about the material.
The actual difference
| Platinum-cure (addition) | Tin-cure (condensation) | |
|---|---|---|
| What is left in the rubber | Nothing that migrates: the catalyst is locked in at trace levels | Organotin catalyst residue; releases alcohol while curing |
| Shrinkage | Under 0.1%, for most grades | Noticeably more, and it continues slightly after cure |
| Ageing | Stable for many years | Dries, stiffens and tears more easily over time |
| Sold as skin-safe | Yes, several grades (Ecoflex, Dragon Skin, Body Double) | No, it is sold for mold making |
| Sensitive to contamination | Very: sulfur clay, latex, uncured resin and more stop the cure | Hardly at all |
| Price | Higher | Lower |
That last-but-one row is the whole trade. Platinum silicone is the better material in every way that matters for a finished piece, but it is fussy: the same catalyst that leaves nothing behind can be poisoned by a latex glove or a resin-printed master. When a platinum cast stays tacky, that is almost always why. Silicone not curing lists every inhibitor and the fix.
How to tell them apart
- Read the technical datasheet (TDS), not the listing. It will say "platinum-cure" or "addition-cure", or "tin-cure" or "condensation-cure". Reputable makers publish one for every product.
- The mix ratio does not tell you. Most soft platinum casting silicones are 1A:1B, but some platinum products are 10:1, and some tin products are 1:1 too.
- Unbranded "silicone rubber" kits from marketplaces are often tin-cure and rarely say so. If the listing and the datasheet do not name the cure type, it is not for this.
- Check the storage date. Silicone has a shelf life, usually about a year unopened, and old part B cures soft. An old kit is a bad place to save money on a body-safe cast.
Choosing a hardness
Silicone hardness is measured in Shore. Soft grades use the Shore 00 scale, firmer ones the Shore A scale, and the two overlap: 00-50 is roughly as soft as Shore A 5 to 10.
| Grade (example) | Hardness | Feel | Working time | Cure |
|---|---|---|---|---|
| Ecoflex 00-30 | Shore 00-30 | Very soft, jelly-like, very stretchy | 45 min | 4 h |
| Ecoflex 00-50 | Shore 00-50 | Soft, a little firmer and tougher | 18 min | 3 h |
| Dragon Skin 10 Medium | Shore 10A | Soft but holds its shape, like a firm gummy | 20 min | 5 h |
| Dragon Skin 10 Slow | Shore 10A | Same, with more time to degas and pour | 45 min | 7 h |
| Dragon Skin 20 | Shore 20A | Firm, springy, very durable | 25 min | 4 h |
Times are at 23 °C from the makers' datasheets; check yours, because they change with temperature and between batches. Softer is not automatically better: very soft grades tear more easily, pick up lint and feel tackier. Shore 10A is a good first choice for a piece that needs to keep its shape, 00-30 or 00-50 for something squishy.
Pigments and additives
Anything you add becomes part of a piece that touches skin, and many common additives also stop platinum silicone curing. The rule is simple: only additives made for platinum silicone, at the dose the maker allows.
| Additive | Use it? | Why |
|---|---|---|
| Silicone pigments for platinum systems | Yes | Made for it. Usually up to about 3% by weight |
| Mica powder | With care | Cosmetic-grade mica works for shimmer; some blue and violet pigments contain compounds that inhibit, so test first |
| Silicone thinner / softener | Sparingly | Makes it softer or thinner, but too much weakens it and can leave an oily surface |
| Acrylic paint, food colouring, alcohol inks | No | Water and solvents disturb the cure and are not skin-safe additives |
| Glitter, craft powders, anything loose | No | Particles at the surface are not sealed and cannot be cleaned |
Getting it bubble-free
Mixing always folds in air, and bubbles at the surface of a finished piece are small pits that are hard to clean. Three ways to get rid of them, from best to cheapest:
Vacuum degassing
Put the mixed silicone in a vacuum chamber. It rises to three or four times its volume, then collapses; hold the vacuum a minute or two after that, then release it slowly. Use a container at least four times the batch volume, and a slow grade so you have time to pour afterwards.
Pressure casting
Pour, then cure the filled mold inside a pressure pot. The pressure shrinks any remaining bubbles until they cannot be seen. It needs a mold that can go in the pot, and it is often combined with vacuum degassing.
No equipment
Pick a slow-cure grade, mix gently, pour from 30 cm up in a thin stream so bubbles burst on the way down, and brush a thin first coat into fine details before filling. Not perfect, but good enough for a first test.
Why a cast feels sticky
This is one of the most searched questions about DIY silicone, and it has two completely different answers.
| What you feel | What it is | What to do |
|---|---|---|
| Sticky and soft, gooey or smeary in places | It did not cure: inhibition or a wrong ratio | Do not use it. Find the cause and cast again |
| Fully firm, but grabby and picks up lint | Normal surface tack of soft grades | Dust with cornstarch, use a matte mold surface, or go firmer |
| Firm, with an oily film | Too much thinner, or release agent left on it | Wash with warm water and mild soap; use less thinner next time |
And for the mold itself?
The mold does not touch skin, so tin-cure is tempting there because it is cheaper and does not care about contamination. The catch: tin-cure residue on the mold surface can stop the platinum silicone you cast into it from curing. For a mold you will cast platinum silicone into, use a platinum mold silicone (Mold Star, Rebound, or similar) and a silicone-on-silicone release every time. How to build that mold, or print a rigid one instead, is in casting silicone into a 3D-printed mold.
Common questions
Is platinum silicone body-safe?
Platinum-cure silicones sold as skin-safe, such as Ecoflex and Dragon Skin, are considered safe for skin contact once fully cured. They leave no catalyst residue, are non-porous and are stable over time. They are skin-safe hobby materials, not certified medical-grade silicone, and only a full cure with nothing foreign mixed in keeps them that way.
Is tin-cure silicone safe for skin?
It is not the material to use for anything that stays in contact with skin. Tin-cure silicone keeps organotin catalyst residue, releases alcohol as it cures, shrinks more and slowly degrades. It is a good, forgiving material for molds, not for a finished piece that touches the body.
How can I tell if silicone is platinum or tin cure?
Read the technical datasheet. Platinum silicone is described as platinum-cure or addition-cure, tin silicone as tin-cure or condensation-cure. The mix ratio is not a reliable tell, because both types come in 1:1 and 10:1 versions. If a product does not say which it is, do not use it for skin contact.
Why is my silicone cast sticky?
If it is sticky and also soft or gooey in places, it did not cure, usually from cure inhibition or a wrong mix ratio, and it should not be used. If it is fully firm but has a tacky feel, that is normal for the softest grades. A light dusting of cornstarch, a matte mold surface or a firmer grade like Dragon Skin 10 all reduce it.
Can I use acrylic paint to colour silicone?
No. Acrylic paint, food colouring and most craft pigments can stop platinum silicone curing and add ingredients that are not skin-safe. Use pigments made for platinum silicone, at the dose the manufacturer gives, usually no more than about 3 percent of the total weight.
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