The measurement of lightfastness is quite a controlled business. There are three main tests used nowadays: ASTM D4303, ASTM (Florida daylight) and ISO 105. The original test was the ISO 105 Blue wool test. That test has been used in the textile industry for many, many years and is the one that Derwent use. ASTM D4303 was developed over the last 15 years in an attempt to create a ligthfastness standard that was applicable to pencils only.
Compare colour changes in comparison to the fading in Blue wool standards 1 – 8.
Blue wool ref 1 will fade very quickly, whereas blue wool ref 8 is considered extremely lightfast. Any result above blue wool 6 is considered to be GOOD and will last in excess of 100 years in gallery conditions.
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As well as lightfastness, we sumbit our pigments to a rigorous colour test using a spectrophotometer, which is an instrument that measures the amount of photons (the intensity of light) absorbed.
The colour test measures colour difference and comprises the following indices:
1. Lightness or darkness
Using a Delta E measurement (how the human eye perceives colour difference) we compare batches with a standard; a score <3 being considered acceptable. Colours are adjusted using this machine and also using the human eye by our highly specialised team.
Every batch of pencils is manually tested for the appropriate texture (softness, waxiness, scratchiness) and, in the case of water-soluble products, its solubility. We test all pencils against a standard for texture.
In the near future we are looking at implementing a further test using a writing machine that will repeatedly use a pencil to test its texture and degree.
A key test is the tensile strength of a pencil core. There are two machines that test this:
1. Hand-operated which measures in Kg and newtons.
2. A tensiometer which measures the tensile strenght of a material. We break points with this machine. This can also be used to measure the breaking point of a pencil held at an angle, the strength of a rubber at the end of a pencil, the tensile strength of the wooden barrel and bonding in the glue.
We break the pencil strips before they are put in wood and ensure they are strong enough.
The flow or viscocity of the paints we use are tested using a special machine. This ensures that the coatings are non-drip and consistent. A centipoise (cP), a dynamic viscosity measurement unit, is obtained which is compared to a standard.
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