Now, the four digits are read as such
The first digit, which is 1, 2 3, 7, or 8 indicates the type of alloy metal added to pure aluminum. In the case of 1XXX, it is unalloyed hence pure aluminum with a purity of 99.00 % or higher.
The second digit, taking this as an example 1Xxx – the blue X, indicated the modification of the alloy used in the combination. For example, the alloy 5657 means the alloy is made from aluminum, silicon, and magnesium together.
The last 2 digits 1XXX – the two blue X are used to correctly indicate the specific alloy and enable easier identification. However, the last two digits in 1XXX signify the purity level of aluminum. For example, 1100 is 99.00 % pure aluminum and 1170 is 99.70 % pure aluminum.