acidic paper 1967:vyp.2(1967:apr.)-1985:vyp.4(1985:okt.) acidic paper pH pen test brittle covers 1969:vyp.2(1969:apr.) brittle covers double fold test--breakage on second fold brittle paper 1968:vyp.1(1968:ianv.)-1969:vyp.2(1969:apr.),1970:vyp.4(1970:okt.)-1972:vyp.1(1972:ianv.),1972:vyp.3(1972:iiul)-1973:vyp.3(1973:iiul),1974:vyp.2(1974:apr.)-1974:vyp.4(1974:okt.),1976:vyp.3(1978:iiul) brittle paper double fold test--breakage on second fold foxed covers 1969:vyp.4(1969:okt.),1971:vyp.1(1971:ianv.)-1971:vyp.2(1971:apr.),1972:vyp.3(1972:iiul),1973:vyp.1(1973:ianv.)-1973:vyp.2(1973:apr.),1974:vyp.1(1974:ianv.)-1975:vyp.1(1975:ianv.),1975:vyp.3(1975:iiul)-1980:vyp.4(1980:okt.),1983:vyp.4(1983:okt.),1985:vyp.1(1985:ianv.)-1985:vyp.2(1985:apr.) stained pages 1968:vyp.4(1968:okt.)
