For many years we carried two products for helping with pigment retention in handmade paper: our Retention Agent and our Scavenger. Both were cationic agents (that is, they developed positive electric charge in water), which attracted them both to the cellulose fibres and the pigments, both of which are generally anionic (developing a negative charge in water). The two products differed in their molecular weight, with the Retention Agent having a fairly low molecular weight and the Scavenger having much higher molecular weight with long single-strand molecules.
Pigment retention is such a complex subject that I really don’t have room to get into it here, except to say that opposite charges attract, so the retention agents either coat the anionic pigment particles making them cationic and thus attracted to the fibres, or the long retention agent molecules cling to both the fibre and the pigment, acting as a tiny thread helping to hold them together. That first effect is why excessive quantities of retention agent actually impair retention because both the pigments and fibres become converted to cationic behaviour.
We started to have trouble purchasing these products in suitable quantities, and being liquid limited their shelf life, so we have now discontinued both of them, replacing both with a powdered retention agent.
To use this new product, you mix some of the powder with water beforehand, which produces a slightly syrupy liquid very much like our old Scavenger in appearance, in usage, and in its chemical properties. The prepared solution has a relatively short lifetime (a few days, depending on water purity and storage conditions), but because you only mix up the amount you need, this means you have fresh solution each time.
There is of course a trade-off between the convenience of having a pre-mixed liquid product and the ease of storage and freshness for a powdered product.
The new product is available in the following sizes and prices (as of February 2020):