A process for making an anchorage additive comprising: 1) combining, at a temperature of -20°C to 160°C, starting materials and, where Starting material is 33% to 67% of a silane composition comprising (A-1) an unsaturated silane of formula R 1 SiR 2 3, where R 1 is a monovalent hydrocarbon group with aliphatic unsaturation, each R 2 is independently selected from the group consisting of a hydrocarbonoxy group and an acyloxy group; (A-2) a trialkoxysilane having an anchorage promoting-group, where the trialkoxysilane has formula R 3 Si(OR 4) 3, where each R 3 comprises an anchorage promoting-group selected from the group consisting of an epoxy-functional group, an acyloxy-functional group, and an acrylate-functional group, and each R 4 is an independently selected alkyl group of 1 to 6 carbon atoms; with the proviso that (A-1) and (A-2) are used in amounts such that a weight ratio of (A-1)/(A-2) has a value of 0.4/1 to 2.5/1; and starting material is 33% to 67% of a polydiorganosiloxane having unit formula: (R 7 R 5 c R 6 (2-c) SiO 1/2) 2 (R 5 2 SiO 2/2) a (R 5 R 6 SiO 2/2) b, where each R 7 is independently selected from the group consisting of OH and OR 4, each R 5 is independently selected from the group consisting of monovalent hydrocarbon groups free of aliphatic unsaturation and monovalent halogenated hydrocarbyl groups free of aliphatic unsaturation, each R 6 is an independently selected monovalent hydrocarbon group with aliphatic unsaturation, subscript a ≥ 0, and subscript b ≥ 0, and subscript c has a value such that 2 ≥ c ≥ 1, where combined amounts of starting materials and add up to 100%. 1) combining, at a temperature of -20°C to 160°C, starting materials and, where Starting material is 33% to 67% of a silane composition comprising (A-1) an unsaturated silane of formula R 1 SiR 2 3, where R 1 is a monovalent hydrocarbon group with aliphatic unsaturation, each R 2 is independently selected from the group consisting of a hydrocarbonoxy group and an acyloxy group; (A-2) a trialkoxysilane having an anchorage promoting-group, where the trialkoxysilane has formula R 3 Si(OR 4) 3, where each R 3 comprises an anchorage promoting-group selected from the group consisting of an epoxy-functional group, an acyloxy-functional group, and an acrylate-functional group, and each R 4 is an independently selected alkyl group of 1 to 6 carbon atoms; with the proviso that (A-1) and (A-2) are used in amounts such that a weight ratio of (A-1)/(A-2) has a value of 0.4/1 to 2.5/1; and (A-1) an unsaturated silane of formula R 1 SiR 2 3, where R 1 is a monovalent hydrocarbon group with aliphatic unsaturation, each R 2 is independently selected from the group consisting of a hydrocarbonoxy group and an acyloxy group; (A-2) a trialkoxysilane having an anchorage promoting-group, where the trialkoxysilane has formula R 3 Si(OR 4) 3, where each R 3 comprises an anchorage promoting-group selected from the group consisting of an epoxy-functional group, an acyloxy-functional group, and an acrylate-functional group, and each R 4 is an independently selected alkyl group of 1 to 6 carbon atoms; with the proviso that (A-1) and (A-2) are used in amounts such that a weight ratio of (A-1)/(A-2) has a value of 0.4/1 to 2.5/1; and