A method of producing polymers of an isolated spider silk protein, comprising the steps of: (i) providing a spider silk protein consisting of from 170 to 760 amino acid residues and comprising: an N-terminal fragment NT consisting of at least one fragment of from 100 to 160 amino acid residues derived from the N-terminal fragment of a spider silk protein, wherein said fragment has at least 50% identity to SEQ ID NO: 8 and/or at least 80% identity to SEQ ID NO: 6 or any individual amino acid sequence in Table 1; and a repetitive fragment REP of from 70 to 300 amino acid residues derived from the repetitive fragment of a spider silk protein, wherein said fragment is selected from the group of L n L, L n AL, L n L, L n GL, wherein n is an integer from 2 to 10; each individual A segment is an amino acid sequence of from 8 to 18 amino acid residues, wherein from 0 to 3 of the amino acid residues are not Ala, and the remaining amino acid residues are Ala; each individual G segment is an amino acid sequence of from 12 to 30 amino acid residues, wherein at least 40% of the amino acid residues are Gly; and each individual L segment is a linker amino acid sequence of from 0 to 20 amino acid residues; and optionally a C-terminal fragment CT of from 70 to 120 amino acid residues, which fragment is derived from the C-terminal fragment of a spider silk protein and has least 50% identity to SEQ ID NO: 9; and/or at least 80% identity to SEQ ID NO: 7 or any individual amino acid sequence in Table 2; and (ii) providing a solution of said spider silk protein in a liquid medium at pH 6.4 or higher and/or an ion composition that prevents polymerisation of said spider silk protein, optionally involving removal of lipopolysaccharides and other pyrogens; (iii) adjusting the properties of said liquid medium to a pH of 6.3 or lower and an ion composition that allows polymerisation of said spider silk protein; (iv) allowing the spider silk protein to form polymers in the liquid medium, said liquid medium having a pH of 6.3 or lower and an ion composition that allows polymerisation of said spider silk protein; and (v) isolating the spider silk protein polymers from said liquid medium. (i) providing a spider silk protein consisting of from 170 to 760 amino acid residues and comprising: an N-terminal fragment NT consisting of at least one fragment of from 100 to 160 amino acid residues derived from the N-terminal fragment of a spider silk protein, wherein said fragment has at least 50% identity to SEQ ID NO: 8 and/or at least 80% identity to SEQ ID NO: 6 or any individual amino acid sequence in Table 1; and a repetitive fragment REP of from 70 to 300 amino acid residues derived from the repetitive fragment of a spider silk protein, wherein said fragment is selected from the group of L n L, L n AL, L n L, L n GL, wherein n is an integer from 2 to 10; each individual A segment is an amino acid sequence of from 8 to 18 amino acid residues, wherein from 0 to 3 of the amino acid residues are not Ala, and the remaining amino acid residues are Ala; each individual G segment is an amino acid sequence of from 12 to 30 amino acid residues, wherein at least 40% of the amino acid residues are Gly; and each individual L segment is a linker amino acid sequence of from 0 to 20 amino acid residues; and optionally a C-terminal fragment CT of from 70 to 120 amino acid residues, which fragment is derived from the C-terminal fragment of a spider silk protein and has least 50% identity to SEQ ID NO: 9; and/or at least 80% identity to SEQ ID NO: 7 or any individual amino acid sequence in Table 2; and an N-terminal fragment NT consisting of at least one fragment of from 100 to 160 amino acid residues derived from the N-terminal fragment of a spider silk protein, wherein said fragment has at least 50% identity to SEQ ID NO: 8 and/or at least 80% identity to SEQ ID NO: 6 or any individual amino acid sequence in Table 1; and a repetitive fragment REP of from 70 to 300 amino acid residues derived from the repetitive fragment of a spider silk protein, wherein said fragment is selected from the group of L n L, L n AL, L n L, L n GL, wherein n is an integer from 2 to 10; each individual A segment is an amino acid sequence of from 8 to 18 amino acid residues, wherein from 0 to 3 of the amino acid residues are not Ala, and the remaining amino acid residues are Ala; each individual G segment is an amino acid sequence of from 12 to 30 amino acid residues, wherein at least 40% of the amino acid residues are Gly; and each individual L segment is a linker amino acid sequence of from 0 to 20 amino acid residues; and optionally a C-terminal fragment CT of from 70 to 120 amino acid residues, which fragment is derived from the C-terminal fragment of a spider silk protein and has least 50% identity to SEQ ID NO: 9; and/or at least 80% identity to SEQ ID NO: 7 or any individual amino acid sequence in Table 2; and (ii) providing a solution of said spider silk protein in a liquid medium at pH 6.4 or higher and/or an ion composition that prevents polymerisation of said spider silk protein, optionally involving removal of lipopolysaccharides and other pyrogens; (iii) adjusting the properties of said liquid medium to a pH of 6.3 or lower and an ion composition that allows polymerisation of said spider silk protein; (iv) allowing the spider silk protein to form polymers in the liquid medium, said liquid medium having a pH of 6.3 or lower and an ion composition that allows polymerisation of said spider silk protein; and (v) isolating the spider silk protein polymers from said liquid medium.