URL: Spectrum 1050 Underglaze Base - Mix 1:2 stain:liquid by weight. Has some frit and additives for suspension, brushing.

https://www.spectrumglazes.com/product/1050-underglaze-base/

Spectrum?s 1050 Underglaze Base is formulated as a base media for preparing your own underglaze colors similar to our 500 series underglazes. The 1050 Underglaze Base has some frit in it, which acts as a flux to help the stain be a compatible fit with either greenware or bisque. It also has various additives, which keep the frit and stain suspended and makes it brush on smooth and easy. It is lead free, non-toxic and dinnerware safe when covered by a dinnerware safe clear glaze.
In order to achieve an underglaze of a strength similar to our 500 series Underglazes we recommend that you mix 1 part of dry powder ceramic stain to 2 parts wet underglaze base by weight. Therefore, for example, if you put half a pint (8 oz. or 228 gm.) of 1050 underglaze base in a container you should mix _ lb. (114 gm.) of stain in this. This recommendation is only a general guideline as the strengths of different color stains vary and you should test colors, particularly the first time you mix them, to determine the correct strength for your application.