diff --git a/masque/file/svg.py b/masque/file/svg.py
index 621bcdb..772aa39 100644
--- a/masque/file/svg.py
+++ b/masque/file/svg.py
@@ -45,6 +45,10 @@ def _make_svg_ids(names: Mapping[str, Pattern]) -> dict[str, str]:
     return svg_ids
 
 
+def _detached_library(library: Mapping[str, Pattern]) -> dict[str, Pattern]:
+    return {name: pat.deepcopy() for name, pat in library.items()}
+
+
 def writefile(
         library: Mapping[str, Pattern],
         top: str,
@@ -53,13 +57,12 @@ def writefile(
         annotate_ports: bool = False,
         ) -> None:
     """
-    Write a Pattern to an SVG file, by first calling .polygonize() on it
+    Write a Pattern to an SVG file, by first calling .polygonize() on a detached
+     materialized copy
      to change the shapes into polygons, and then writing patterns as SVG
      groups (<g>, inside <defs>), polygons as paths (<path>), and refs
      as <use> elements.
 
-    Note that this function modifies the Pattern.
-
     If `custom_attributes` is `True`, a non-standard `pattern_layer` attribute
      is written to the relevant elements.
 
@@ -71,19 +74,21 @@ def writefile(
      prior to calling this function.
 
     Args:
-        pattern: Pattern to write to file. Modified by this function.
+        library: Mapping of pattern names to patterns.
+        top: Name of the top-level pattern to render.
         filename: Filename to write to.
         custom_attributes: Whether to write non-standard `pattern_layer` attribute to the
             SVG elements.
         annotate_ports: If True, draw an arrow for each port (similar to
             `Pattern.visualize(..., ports=True)`).
     """
-    pattern = library[top]
+    detached = _detached_library(library)
+    pattern = detached[top]
 
     # Polygonize pattern
     pattern.polygonize()
 
-    bounds = pattern.get_bounds(library=library)
+    bounds = pattern.get_bounds(library=detached)
     if bounds is None:
         bounds_min, bounds_max = numpy.array([[-1, -1], [1, 1]])
         logger.warning('Pattern had no bounds (empty?); setting arbitrary viewbox', stacklevel=1)
@@ -96,10 +101,10 @@ def writefile(
     # Create file
     svg = svgwrite.Drawing(filename, profile='full', viewBox=viewbox_string,
                            debug=(not custom_attributes))
-    svg_ids = _make_svg_ids(library)
+    svg_ids = _make_svg_ids(detached)
 
     # Now create a group for each pattern and add in any Boundary and Use elements
-    for name, pat in library.items():
+    for name, pat in detached.items():
         svg_group = svg.g(id=svg_ids[name], fill='blue', stroke='red')
 
         for layer, shapes in pat.shapes.items():
@@ -158,21 +163,21 @@ def writefile_inverted(
      box and drawing the polygons with reverse vertex order inside it, all within
      one `<path>` element.
 
-    Note that this function modifies the Pattern.
-
     If you want pattern polygonized with non-default arguments, just call `pattern.polygonize()`
      prior to calling this function.
 
     Args:
-        pattern: Pattern to write to file. Modified by this function.
+        library: Mapping of pattern names to patterns.
+        top: Name of the top-level pattern to render.
         filename: Filename to write to.
     """
-    pattern = library[top]
+    detached = _detached_library(library)
+    pattern = detached[top]
 
     # Polygonize and flatten pattern
-    pattern.polygonize().flatten(library)
+    pattern.polygonize().flatten(detached)
 
-    bounds = pattern.get_bounds(library=library)
+    bounds = pattern.get_bounds(library=detached)
     if bounds is None:
         bounds_min, bounds_max = numpy.array([[-1, -1], [1, 1]])
         logger.warning('Pattern had no bounds (empty?); setting arbitrary viewbox', stacklevel=1)