The Silicone Hardness Figure Nobody Publishes

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No, because durability is not what the phrase is about. Food grade is a compliance statement: it says the finished rubber has been assessed against the limits for what can migrate out of it under defined conditions of repeated contact. It is a statement about chemistry. It is not a stiffness, not a tear strength, not a temperature rating and not a thickness, and a compound can hold the compliance and still be the wrong rubber for the job in every mechanical respect.

The figure that decides whether a silicone article works is Shore A hardness, and it is published almost nowhere. Across every silicone product on this site there is not one. So the useful half of this post is not a buying recommendation, it is a pair of tests you can do with your hands, and the arithmetic that says why they work.

Two words that get read as one

Our YESCOO silicone griddle mat is the clearest example of the conflation, and not because the listing is unusually bad. It is typical. Its specification table has a row labelled Material reading food grade silicone and a row labelled Heat reading high temperature resistant. One row is a compliance claim and the other row is an adjective where a number belongs, and because they sit next to each other they get read as one fact about a good material.

They are unrelated. Compliance testing looks at extractables. Temperature capability depends on the cure system and the filler package: an addition cured silicone and a peroxide cured one can hold identical compliance and differ in what they tolerate, and neither cure system is ever named on a consumer listing. Our own record is honest about the limits of what is being claimed, with a drawback line saying the mat complements rather than replaces normal heat precautions and is not a limitless shield, which is the right note to strike. More in the YESCOO griddle mat review.

What Shore A actually measures

The test is ASTM D2240 and it is almost crude. A shaped indenter on a calibrated spring is pressed against the rubber, and the instrument reports how far the indenter sinks in. The scale is defined so that zero penetration reads 100 and 2.54 millimetres of penetration reads 0, and everything in between is linear. So one point on the Shore A scale is exactly 0.0254 millimetres of indenter travel. Shore A 20 is 2.03 millimetres of penetration, Shore A 50 is 1.27, and Shore A 80 is 0.51.

That is a depth, not a stiffness, and the relationship between the two is where everything interesting happens.

Convert each hardness to a Young's modulus using Gent's standard relation and the numbers come out like this. Shore A 20 is about 0.73 megapascals. Shore A 50 is about 2.46. Shore A 80 is about 9.35. Shore A 90 is about 21. So moving 60 points up a 100 point scale multiplies stiffness by nearly 13 times, and the last ten points multiply it again by more than two. The scale looks like a percentage and behaves like an exponent, which is exactly why a figure in the forties and a figure in the seventies describe materials that behave nothing alike while sounding like neighbours.

A drain stopper, and the number that decides whether it seals

Our silicone shower drain hair stopper, in a two pack describes itself as soft silicone that adapts to slightly uneven drain surfaces, and lists as drawbacks that the suction fit varies on uneven surfaces and that very textured drains may not seal well. Those two statements bracket the whole problem, and the arithmetic sits inside them.

A suction seal has exactly one force available to press the lip into the irregularities of the drain surround: the difference between atmospheric pressure outside and the reduced pressure under the cup. At best that is about 101 kilopascals, and in practice much less. Now ask what stress the lip needs in order to conform. Suppose the surround has a 0.2 millimetre bump over a 2 millimetre span, which is an unremarkable amount of texture on a tiled or cast surface. The lip must strain locally by roughly 0.2 over 2, which is 10 percent, and the stress required is the modulus multiplied by that strain.

At Shore A 20 that is 0.73 megapascals times 0.1, which is 73 kilopascals. Atmospheric pressure can supply that, with a little in hand. At Shore A 80 it is 9.35 megapascals times 0.1, which is 935 kilopascals, or about nine atmospheres. There is no suction cup in the world that will deliver nine atmospheres, so a hard lip simply bridges the bump and leaves a path around it, and the water finds it.

Same geometry, same drain, same product shape. One unpublished number decides whether it works, and the margin is a factor of nine on a force that is capped by the atmosphere. That is why the only honest thing a listing can say about a suction seal is what ours says: that it varies with the surface. Record in the silicone drain stopper review.

A bag seal, and the thumb that has to close it

Our Haakaa reusable silicone food storage bags, a three pack close with what the listing calls an interlocking dot seal, in 500, 1000 and 1500 millilitre sizes. An interlock works by interference: one moulded feature is slightly larger than the groove it goes into, and closing it means deforming the rubber by that difference along the length of the seam.

The force to do that is proportional to the modulus, which gives a very clean ratio. If the interference is the same, the same seam at Shore A 70 needs 5.52 divided by 1.69, which is about 3.3 times the force of the same seam at Shore A 40. A thumb run along a 20 centimetre seam has a fixed budget: comfortable sustained pressure, a small contact patch, and one pass. Triple the required force and the seam closes in some places and not others, which is a bag that looks shut and leaks in a freezer drawer.

So the softer compound makes a better hand seal, and the trade is that it also deforms more easily under its own contents, which is why the same listing notes the bags stand upright thanks to a sturdier base, and that is geometry doing the job hardness cannot. Record in the Haakaa silicone food storage bags review.

A mat, and why thickness beats hardness anyway

Here is the part that makes the missing number less tragic than it sounds.

Whether a sheet lies flat or curls is a bending problem, and the bending stiffness of a sheet goes as the cube of its thickness. Double the thickness and bending stiffness goes up eightfold. Go from one millimetre to three and it goes up twenty sevenfold. Compare that with the 12.8 times you get from running the entire span of the practical hardness range, from Shore A 20 to Shore A 80, and thickness is the bigger lever by a comfortable margin.

Work the comparison directly. A one millimetre sheet at Shore A 80 has a flexural rigidity per unit width of about 0.00104 newton metres. A two millimetre sheet at Shore A 20, despite being the softest rubber on the chart, comes out at about 0.00065, so it is still the floppier of the two. Take that soft sheet to three millimetres and it reaches about 0.0022, which is more than twice the rigidity of the hard thin one. Thin and hard behaves like stiff and crinkly; thick and soft behaves like heavy and obedient.

Which lets you read our Vodolo silicone griddle tool caddy mat properly. Its specification table lists Material as high quality durable silicone and Size as large, holds a full tool set, and neither of those is a specification: one is an adjective and the other is a use case. There is no thickness, no hardness and no temperature. The record does flag the consequence honestly, saying to keep it within silicone's heat limit and that it is not a hot pan pad, while never stating what that limit is, because the listing does not. For a mat whose whole job is to lie flat under a set of tools without creeping, thickness is the figure you want and it is absent. Record in the Vodolo griddle mat review.

The same arithmetic also explains why the YESCOO mat above is described as both high temperature resistant and thin and foldable, which pull against each other. Silicone conducts heat at roughly 0.2 watts per metre per kelvin, so a two millimetre sheet has a thermal resistance of about 0.01 square metre kelvins per watt. That is not insulation by any useful measure; it is about a hundredth of what a thin camping mat gives you. A thin mat protects a surface by being non combustible, by catching grease and by introducing a tiny air gap, not by being a thermal barrier, and treating it as a barrier is exactly the mistake the listing's thin and foldable selling point invites. Our record is right to call it not a limitless shield.

One more place the word soft is doing the wrong job

Our linseyCove makeup brush cleaner soap with a silicone pad is worth reading as a specification puzzle. Its table lists the pad texture as low friction, ridged and grooved, and the feature text says a low friction pad texture cleans thoroughly while keeping brushes soft. A cleaning pad works by shear: you drag loaded bristles across a surface and the surface has to resist them enough to peel the product off. Low friction is the opposite of that requirement, and the listing is selling the pad on the property that works against its function while claiming the function in the same sentence.

What it almost certainly means is gentle, and what makes a textured pad gentle rather than destructive is not the friction coefficient at all. It is the hardness of the rubber and the radius of the ridges. A soft pad with generous radii deflects around a bristle bundle and loads it over a wide contact; a hard pad with sharp ridges concentrates the same hand force onto a line and breaks fibres. Neither the hardness nor the ridge radius is published, and those are the two numbers that decide it. Record in the linseyCove brush cleaner review.

The honest limit

Not one silicone product on this site publishes a Shore A hardness, a thickness, a tear strength, an elongation at break or a cure system, and several publish adjectives in fields that are labelled as specifications. So there is no better listing to go and find, and asking a seller for a durometer figure on a drain cover will not get you one. The arithmetic in this post is standard rubber engineering applied to published geometry, not a measurement of any product here.

What is left is a hand test that costs nothing and is surprisingly discriminating, because of the exponential curve in the drawing above.

Press a thumbnail into the article. If the nail sinks in easily and the dent springs back slowly, you are somewhere in the twenties or thirties, which is the right end for anything that has to seal against an imperfect surface or be pressed shut by hand. If the nail barely marks it and the surface feels like a tyre tread, you are in the seventies or above, which is right for anything that has to hold a shape or resist a tool and wrong for anything that has to conform. The middle feels like a pencil eraser. Three categories, one thumbnail, and because stiffness changes by more than twelve times across that span, the categories are genuinely far apart rather than a matter of taste.

Then measure the thickness, because that is the lever that matters more and the one you can actually check. A ruler against the edge, or two fingers and a guess, will separate a one millimetre sheet from a three millimetre one immediately, and the difference between them is a factor of twenty seven in whether it lies flat. For a mat, thick and soft beats thin and hard. For a seal, thin and soft beats anything hard. For a scraper or a tool rest, hard beats soft and thickness barely matters.

And the limit worth keeping in view: for a drain cover or a tool mat the stakes are a small annoyance, and the free fix for a stopper that will not seal is usually not a better stopper. It is cleaning the drain surround so there is less texture to bridge, which removes the problem from the equation entirely rather than asking the rubber to solve it. Geometry first, then thickness, then the number nobody prints.