Reference for Tahorot 1:8
מִקְרֶצֶת שֶׁהָיְתָה תְחִלָּה וְהִשִּׁיךְ לָהּ אֲחֵרוֹת, כֻּלָּן תְּחִלָּהּ. פֵּרְשׁוּ, הִיא תְחִלָּה וְכֻלָּן שְׁנִיּוֹת. הָיְתָה שְׁנִיָּה וְהִשִּׁיךְ לָהּ אֲחֵרוֹת, כֻּלָּן שְׁנִיּוֹת. פֵּרְשׁוּ, הִיא שְׁנִיָּה וְכֻלָּן שְׁלִישִׁיּוֹת. הָיְתָה שְׁלִישִׁית וְהִשִּׁיךְ לָהּ אֲחֵרוֹת, הִיא שְׁלִישִׁית וְכֻלָּן טְהוֹרוֹת, בֵּין שֶׁפֵּרְשׁוּ בֵּין שֶׁלֹּא פֵרְשׁוּ:
A piece of dough that was primary [i.e. of a first degree impurity], and one attached other pieces to it, they are all primary; if one [then] separated them, it [the original piece] is primary, and all the others are second degree. If it [the original piece] was second degree and one attached other pieces to it, they are all second degree; if one [then] separated them, it is second degree, and all the others are third degree. If it was third degree and one attached other pieces to it, it is third degree, and all the others are pure, whether they were separated or not.