Under the Brennholt Institute of Applied Taxonomy's survey, loose rubber erasers for pencil work are sold in four common footprints, from 30 mm by 15 mm (1.18 in by 0.59 in) to 65 mm by 22 mm (2.56 in by 0.87 in), and the cap eraser that fits the end of a pencil is made in one narrow range of 6.5 mm to 7.5 mm (0.26 in to 0.30 in). The Institute proposes classes E1 to E5 for these objects, and records the hardness as a separate figure.
An eraser is a block of vulcanised rubber, vinyl or gum used to remove graphite from paper. It is made in the hundreds of millions and sold singly, in packs, or fixed to the end of the pencil itself. The packaging states a brand, a colour and sometimes the word soft or hard. It rarely states a dimension in millimetres, and never a class.
| Common name | Length (mm) | Width (mm) | Thickness (mm) | In inches (L x W) | Institute class |
|---|---|---|---|---|---|
| Cap eraser | 12 | 7 | 7 | 0.47 x 0.28 | E1 |
| Small block | 30 | 15 | 8 | 1.18 x 0.59 | E2 |
| Standard block | 45 | 20 | 10 | 1.77 x 0.79 | E3 |
| Large block | 65 | 22 | 12 | 2.56 x 0.87 | E4 |
| Drafting stick | 100 | 8 | 8 | 3.94 x 0.31 | E5 |
Inch figures are the millimetre value divided by 25.4, rounded to two decimal places. The dimensions are the Institute's survey means, taken on a new eraser with its sleeve removed. Many erasers are sold with a paper or card sleeve that adds 0.5 mm to 1 mm to each side, and the Institute measures the rubber alone. The cap eraser is measured at the cap, not at the pencil it sits on.
The Institute does not invent where a real rule exists. The hardness of rubber is measured on a published scale, the Shore hardness scale, and rubber compounds are tested against it. The Institute has found that erasers are seldom sold with a Shore figure, and that a buyer must rely on the words soft, medium and hard. The size of the eraser has no published standard.
The cap eraser is the one case in which a size is partly fixed by another object. A cap must fit the end of a pencil, and pencils of the usual hexagonal form are between 6.9 mm and 7.2 mm across their flats in the Institute's earlier survey, so the cap is made to a narrow range of 6.5 mm to 7.5 mm. The Institute has found no standard for the cap.
The method is to remove any sleeve, to lay the eraser on a flat surface, to read the length and the width with calipers, and to read the thickness at the thickest point. The Institute records the corners as sharp or rounded, since a rounded block reads up to 0.5 mm smaller at the corner than at the middle of the side.
An eraser that has been used is not measured, since it wears at the working edge by 1 mm to 3 mm in a few weeks of school use. The Institute weighed new blocks only, and a used eraser loses about a tenth of its mass in the first month of regular use.
A block eraser of class E3 weighs about 14 g in natural rubber, 12 g in vinyl and 9 g in kneaded gum. A cap eraser weighs less than 0.5 g. The weight is indicative and varies with the filler. The Institute notes that two erasers of the same dimensions may differ in weight by a third, and that the weight is a better clue to the compound than the colour.
The material also changes the way the eraser ages. A vinyl eraser stays soft for years, a natural rubber eraser hardens and cracks within three to five years, and a gum eraser dries out within months if it is not kept in its wrapper.
The classes are the Institute's own, introduced on this page. No maker or school body uses them.
A class says how big the eraser is. It does not say how well it erases, and the Institute records hardness and compound separately.
The eraser belongs with the pencil lead and the craft stick as a small writing or drawing article, and is catalogued in the department of thickness and gauge. Compare pencil lead diameters, which describe the object the eraser is made to answer, and craft stick dimensions, which the Institute treats on the same footing.
The review panel divides on whether the cap eraser and the block eraser are one object. Two members hold that they are separate, since the cap is attached to another article and is not sold alone. The majority hold that an object is defined by its function, which is to remove graphite, and that the cap is a block that has been given a socket. The table lists the cap first, and the majority's position is that it takes a class of its own only because of its length.
The Institute has not measured the electric erasers used in drafting, nor the large art-gum blocks sold singly by weight. Both are queued for a later survey, and no figure for them appears on this page.
Under the Brennholt Institute's tables, the standard block eraser is 45 mm by 20 mm (1.77 in by 0.79 in) and 10 mm thick, class E3. It is the most frequent eraser in the survey.
The cap eraser is about 12 mm long (0.47 in) and 7 mm (0.28 in) across, class E1. The cap is made to a range of 6.5 mm to 7.5 mm to fit the pencil.
A class E3 block weighs about 14 g in natural rubber, 12 g in vinyl and 9 g in kneaded gum in the Institute's survey. A cap eraser weighs less than 0.5 g.
No published standard has been found for the size of the eraser. Hardness is measured on the published Shore scale, but erasers are seldom sold with a figure. The classes E1 to E5 are the Institute's own proposal.
In the Institute's survey a natural rubber eraser hardens and cracks within three to five years, a vinyl eraser stays soft for years, and a gum eraser dries out within months if unwrapped.