Firmness refers to the hardness of the silicone being used, not the firmness of the finished toy.** In the indie adult novelty world this is measured using the shore 00 and shore A hardness scales.

Shore Hardness comparison chart from Smooth-On

While you will only see the Shore 00 and A scales in indie toys; there are actually 12 scales described in ASTM D2240-15, other measurement protocols such as those described in ASTM D1415-18, and entirely independent scaling systems such as Rockwell Firmness which is used for rigid plastics and metals. 

Shore firmness is measured by recording the force needed to indent a material to a specific depth with a tool. You can think of it like taking a sharpened pencil and measuring how hard it is to push the tip of the pencil 2.5mm into the material. Each scale is measured from 0-100. If it is hard to push the indenter into the material it will get a higher number on that scale, and if it is easy then it receives a lower number. To change scales, you change the shape of the indenter. Moving from a firmer scale like a D (used for hard rubbers and plastics) to A (used for soft rubber) would mean changing from a sharpened pencil to a dull pencil. And going from the A scale to the 00 Scale (Used for super soft rubbers and soft foams) would mean changing from a dull pencil to an unsharpened pencil.

Photo shows indenters and their measurements. From miconveyancesolutions

Reported firmness in the technical documentation of a silicone is not particularly accurate to how the silicone will truly "feel" when squished in the hand, particularly with super soft silicones. The shore hardness scale is much more of a tool for comparison and conversation than it is an accurate and repeatable scientific measurement for toys. As an example, Dragon Skin 10 SLOW and Dragon Skin 10 NV are both materials with firmness of 10 on the A scale (10A), but Dragon Skin 10 SLOW is very noticeably firmer than Dragon Skin 10 NV. For more information on Shore firmness, Smooth-On has a page HERE

Firmness in Indie Adult Novelty

Silicones used in the indie adult novelty industry are much softer than what is common in the wider adult novelty world. They tend to be closer to what one would expect from a "Jelly Toy." These very soft silicones are measured on the A or 00 Shore hardness scale, while most of the silicones that people are familiar with from lids, containers, and cooking utensils falls between the high A and low D scales.

Toys are normally described as extra soft, soft, medium, etc. rather than using their shore firmness in the listing. Often the shore hardness is included in addition to this somewhere in the listings or FAQ. The ends of the spectrum - Extra Soft and Extra Firm are highly variable from shop to shop but the middle firmness options *tend* to be:

• Soft: 00-30 (Shore 00 scale firmness 30)
• Medium: 00-50 (Shore 00 scale firmness 50)
• Firm: 10A (Shore A scale firmness 10)

Twitter user @KateVulpes has taken the time to compile and maintain a list containing many shops and their known firmnesses along with notes HERE

Photo shows screen shot of Kate's chart

While many shops use the same firmnesses and same materials, these comparisons should always be taken with a grain or two of salt. Besides the previously mentioned Dragon Skin 10 SLOW vs 10 NV, there are a plethora of things that impact firmness.

Amavidi: Silicone Firmness - How is Firmness measured?

What Impacts Firmness

• Size: Larger toys feel firmer than smaller toys
• Shape and Details: Various effects
• Pigments: Generally higher pigment load feels firmer
• Addition of Silicone Thinner: More thinner in a toy feels softer
• Addition of PlatCat: More PlatCat feels firmer
• Addition of Other Additives: Various
• Cure Conditions: Various, only slight impact
• Exposure to Heat: Toys that have been heated become slightly firmer, Near Clear 00-31 being the exception as it experiences significant firming with long-term heating, becoming closer to 00-50 than 00-30

What is the best firmness to start with?

There is no best firmness to start with. The majority of customers divide up between 00-30 and 00-50, making this the largest slice of the market; but customers preferring super soft, firm, and extra firm options are chronically underserved. Additionally, much of what impacts the best firmness to start with is really a conversation about the best material to start with. Many aspects of choosing first materials relate to how easy the material is to process, rather than which one will have a slight advantage on sales.

That said, a short thought experiment may be helpful when choosing between Smooth-On's Ecoflex 00-30 and Ecoflex 00-50 (as these are both available to most makers)

Imagine you are driving down the highway and very hungry. As the next exit comes up you see a sign showing what food options are at the exit out of the corner of your eye. A few moments later you speed by the exit and the option to stop is lost. Did you miss the exit because?

A. You made an executive decision to keep driving and see what's at the next exit

B. You experienced analysis paralysis and missed it while still deciding what to do

If you answered (A) then start with Ecoflex 00-50. It is a little more viscous and gets some great marbles for starters, while being less likely to have significant rub out from heavy pigments settling during cure like GITD. But, it is also has a less forgiving pot life and the cure kinetics become quickly nonlinear. This means you will only have a few minutes of warning time between when it is noticeably more viscous and when it becomes an unusable bucket of trash. With 00-50, in any kind of warm climate, you will end up making those executive decisions to get the silicone out of the pour containers and into any available mold.

If you answered (B) then start with Ecoflex 00-30. It has a longer pot life and much more predictable and linear-feeling cure kinetics. You will be able to see as it slowly goes from a thin liquid to a thicker gel. Its lower viscosity makes it less prone to overflowing the containers while being degassed, and makes pouring thin, controlled streams easier. However, the low viscosity allows heavier pigments to settle out, making GITD and Inclusion pours prone to flops; as well as allowing density-based mixing to occur during cure, which can destroy butt art and cause muddy colors.