Blue on a Sanborn Map? Read the Label Before Calling It Stone

A blue building on a Sanborn fire insurance map does not have one universal translation. In a key preserved by the Library of Congress, blue appears beside several different construction descriptions. The letters inside the colored shape matter.

For a collector describing an old map, that is a useful distinction: you can explain what its symbols record without turning a color into an unsupported claim about a building. Start with the key belonging to the edition you are reading. The example below shows why.

One color, four different entries

Open the Library of Congress's full Sanborn key and look at the upper-left panel. Its blue examples include:

• A stone-building entry
• C. BR., expanded as concrete, lime, cinder or cement brick
• C. B., used for hollow concrete or cement-block construction
• CONC., used for concrete or reinforced-concrete construction

These are readings of this particular key, not a codebook for every Sanborn edition. Their shared color supplies less information than the color and inscription together. A small, legible abbreviation can therefore change the description you write.

The source is a reference sheet, not an examination of a particular house. It does not establish which materials survive in a building today.

Why two quick guides can sound different

The Library's 2017 discussion of an 1898 Casa Grande map describes blue as stone. Its Sanborn interpretation guide discusses blue for concrete and cinder-block construction, while also warning that symbols changed and were applied in different ways.

Those short explanations should send you back to the relevant map and its key. They are not permission to replace every blue shape with whichever material you read about first. The full reference sheet makes the problem visible: several labels share a blue background.

There is another easily missed distinction. On a graphic index, color may divide the town into the areas covered by different sheets. Before interpreting a colored region as a building material, establish whether you are looking at a detailed building plan or a sheet-location map. The Library's guide explains both uses.

A three-pass reading exercise

The following is an invented exercise using the key above. It is not a report about an identified property.

Imagine a cropped map image showing a blue rectangle. A seller's caption calls it a stone house.

1. At first, only the color is readable. You can record “blue building symbol.” The construction description remains unresolved.
2. In a clearer image, C. B. becomes legible. If the map's own key matches the reference used here, the relevant description is hollow concrete or cement-block construction. The earlier stone caption needs reconsideration.
3. The full sheet turns out to belong to a different edition whose key has not been found. Keep the letters and color in your notes, but suspend the expansion. A familiar-looking symbol is not enough to establish that this key applies.

The useful result is a decision about wording. You have identified exactly what would let you make the description more specific: a readable inscription and the matching legend. You have not diagnosed the real building from a colored rectangle.

Find the matching sheet, not just a similar picture

Record the place, edition date, volume and printed sheet number before searching. The Library's Sanborn search walkthrough shows why a place name alone can produce the wrong city. State, county and city filters help narrow the result; smaller communities may appear under a neighboring town or a secondary location.

Keep the detailed sheet and its introductory material together in your collection record. A detached sheet can be attractive on its own, but a link to its matching key makes it more useful to the next reader.

For broader historical research, symbols are only one entry point. In a Library of Congress research example from Erie, a building's printed church name supplied a lead that a general web search had missed. Read names and annotations as well as colors.

Write a description that stays with the evidence

A useful caption might read: “The map shows a blue building symbol labeled C. B.; in the matching key, this denotes hollow concrete or cement-block construction.” Include the sheet and key links beside it.

If the matching key is missing, shorten the claim: “Blue building symbol; C. B. is visible, but its meaning in this edition has not been verified.” That is still a useful observation. It gives the next researcher a concrete question instead of an inherited guess.

For a different kind of evidence problem, our painting-provenance guide explains how to keep a source's statement separate from what it establishes about an object.