Metal-containing colloids are manufactured by reacting a plurality of metal ions and a plurality of organic agent molecules to form metal complexes in a mixture having a pH greater than about 4.25. The metal complexes are reduced for at least 0.5 hour to form stable colloidal nanoparticles. The extended reduction time improves the stability of the colloidal particles as compared to shorter reduction times. The stability of the colloidal particles allows for colloids with higher concentrations of metal to be formed. The concentration of metal in the colloid is preferably at least about 150 ppm by weight.